Too long DNR:
David Clark Pro-X ANR/ENC (Winner! Convenient and comfortable)
Bose ProFlight (Best features, excellent sound quality)
Clarity Aloft Flex (Not much nice to say about it)

I'll start with the disclaimers. Zero scientific measurements taken, and as you will read further the comparison really boiled down to comfort and ease of use with features taking a back seat. All 3 sounded great and offered plenty of sound suppression in a fairly quiet cockpit (717). My goals were (in order) comfort, sound quality, hearing protection, ease of use, and extra features. None of the test legs were over 2 hours and other than owning and loving other Bose products and disliking the color green, I had no agendas (or sponsors) going in. I paid for all 3 headsets with the intent of returning the losers minus shipping costs. It was my intent to purchase all 3 from the manufacturer since Bose and DC have airline pilot discounts but availability was an issue so I had to get the Bose on Amazon and the DC from Marv Golden Pilot Supplies. I won't spend too much time on physical descriptions since each company's web site will do a better job; however, I will talk about the implications of those design decisions. I'll list the headsets in order that I received them.

Clarity Aloft Flex $710 ($725 + $15 shipping - $30 SUNnFUN discount)
Very light headset with a small carrying case which easily fit into my bag. I wore yellow foamies in the USAF for 20 years so the earplug design didn't bother me; however, when compared with the other 2 it was less comfortable. Sound quality was great and noise attenuation was probably the best of the 3. Completely passive system which doesn't require batteries another plus. After this, I run out of nice things to say about this headset.

Of the 3, it's the most cumbersome to put on -- there is very little "form" to the headset with wires dangling everywhere. It easily gets tangled on itself and I often found myself fumbling around trying to locate the speakers in order to insert them into my ear. If you're long haul maybe less of an issue, but on a NB where you might need to change planes once or twice per day plus multiple opportunities to speak with ground personnel, MX, or the company it's a significant issue. They include several foam covers for each speaker and I needed them, in only 2 legs of use I had a foam cover fall off 3 times and lost one of them. Perhaps I put them on incorrectly but even if I did I still blame the manufacturer: it's either a poor design that doesn't stay adequately secure or it's a poor design that leads the user to think they've properly installed the foam cover but haven't. Cockpit conversations were difficult with the earplug fully inserted so you have to loosen it a little bit to talk to the other pilot but unlike the other 2, once you want to go back to full noise attenuation you have to fully remove the ear piece, squish it up, and reinsert. This in and out gives the troublesome foam covers additional opportunities to work themselves loose. The speakers are held in place with an over the ear bracket similar to what several manufacturers use with running headsets. This design copies what you find in the ultra-cheap running headset category and after 30 minutes it developed pressure points on my ear cartilage making this a VERY uncomfortable headset for me. This was also the only headset that interfered with my movement -- the cable or ear bracket kept getting caught on my left shoulder harness whenever I turned my head left. While the sound quality was excellent, outgoing transmissions were terrible. The boom mic (which fell out of its housing several times) is not long enough and I had to constantly readjust the mic for ATC to be able to hear me. Not a big deal for me but if you care there is no Bluetooth, only a 3.5mm receptacle. Finally, it's a 3D puzzle trying to get the headset back into its case without putting any pressure on the wire connections. Due to its size and weight I really wanted to love this headset but after testing there was no way I would ever consider owning this. Others on this board swear by this headset so as usual YMMV.

Bose ProFlight $1069 ($996 + $69 tax)
I love Bose products; I own a pair of QC25s, QC35s, and 2 pairs of Soundsport running headphones so I'm intimately familiar with their earpiece design and sound quality. The first thing you notice when you get these (aside from big $$$) is the gigantic hard case, if you are commuting this will be a major negative and even though I live in base I didn't like giving up such a huge portion of my flight bag to the ginormous case (I'd estimate about double the volume of a QC35 case). Of the 3, I felt like the sound quality was the best and the active noise cancellation is adjustable making it a run away winner in this category. The Bluetooth was simple to pair and the headset has a function which will mute your connected device for any incoming radio transmission. I tested this feature at the gate and it worked perfectly. The battery case has a clever rubber loop around it giving you a great way to hang the weighty control module. I also like that Bose considered you might want to speak with the other pilot. You can remove the earpiece entirely and there is a built-in hook to hang the speaker which works very well. They also have a tap function which will turn off noise cancelling in just one ear piece. I found that feature to be clunky and unusable. It hurt my hearing to tap on the speaker in my ear and it usually took several taps to get it to work. Once noise cancelling was turned off I found the sensation of cancelling in one ear and none in the other to be "odd" at best. Good idea in theory but in practice I'd never use it.

In regards to comfort, I found it to be OK. I couldn't find the sweet spot where it didn't apply too much pressure to my noggin but wouldn't slip when I turned my head. It wasn't so uncomfortable as to cause pain, but I definitely noticed the pressure above my ears from the mounting system. If I got it where there was minimal pressure above the ears, then due to the heavy weight of the cord and battery compartment, when I'd turn my head the headset would slide back and forth on my head. Not a deal breaker but not ideal. The ear pieces are very similar to my running headset and just like the running headset, they occasionally move around. They never fell out of my ears but I had to readjust them several times in-flight as I felt them working their way out. Storing them or putting them on quickly between flights wasn't anywhere near as bad as the Clarity Aloft, but it also wasn't anywhere near as convenient as the DC Pro-X. I could see owning a pair of these, but in the end they weren't my favorite of the 3.

David Clark DC Pro-X ANR/ENC: $695
These are the personal headsets I see the most often on the 717 and now I see why. This is the only headset that ships with a soft container which I prefer. Donning the headset and taking it off as well as storage was extremely simple which is great for a NB pilot. I feel like sound quality and noise attenuation wasn't quite as good as the other 2 headsets, but all 3 are so much better than what I was used to the small difference between the 3 was (literally and figuratively) in the noise. Maybe if you're on a 737 the noise cancelling might be borderline and you'd want something over the ear but on anything else it should be more than enough. If you care about the Bluetooth, it took a while for my iPhone to find the DC but once it did I had no issues. I also tested this pair at the gate and while there is no mute feature, you have separate volume controls for incoming transmissions and Bluetooth. You also have independent volume control for each ear which would be great if you had more hearing loss in one of your ears, or you want one side lower to speak with the other pilot. Since this is the only pair that goes on top of the ear, in a quiet cockpit you really don't even need to turn on the noise cancellation for adequate levels of protection; however, I had it on for my test. Talking with the other pilot could be done with active noise cancellation or you can go old-school and just slip one side halfway off. The biggest selling point of these headset was the comfort -- the headset had no pressure points and stay in place when I turned my head side to side. Combine the best comfort, the most convenience, comparable sound quality, adequate noise cancellation, and over $350 less than the Bose and this was a run away winner.