The "differences pay" for a new hire, is, as mentioned above, about $250/mo. This was one of the reasons I chose the XL/XLS.
When I interviewed, I had the choice of about 5 different aircraft. It made a big difference in how soon I started according to what airplane is chosen. NetJets is happy enough to give you whatever plane is available, but class schedules are pretty much set quite a bit in the future. If they know they'll have a new-hire slot in the Falcon, but it won't be for six weeks, you're going to have to wait. Conversely, if they discover a slot the next week, it will be on a particular aircraft.
As far as aircraft go: I have type ratings on six different airplanes and experience in lots of others. While the Citation XL is a great aircraft for the company and our customers, I would have to say, it is without a doubt the worst airplane I've ever flown. How Cessna got it certified is a mystery to me. I like the schedule, I like the captains, I like the length of legs, but I hate the plane. It is totally "rinky-dink". Most of the guys who like it, don't seem to have much to compare it to.