I used a 750 for years, and i liked it very much, but I was wearing ear-buds underneath. If your pilot-in-crime is a low-talker or mumbler....get used to saying a lot of: "say again", "excuse me", "uh hu", "ha ha", and "altitude select"!
I traded another pilot for an 850 for a leg, and I did not really like it. I just liked my 750 set up better and was used to it. The 850 was louder than the 750 with ear-buds. So, if you like your 850 I would give it a run, but would still recommend ear-buds underneath. I know that sounds annoying, but you get used to it.