Only the right angle chuck, offset chuck and the drywall chuck are not interchangeable between the CXS and the other Festool drills.

I'm sure Festool had a good reason, but I think a lot of people would have preferred the CXS's keyless chuck to go to 3/8" instead of 5/16" / 8mm.  At least you can use the larger keyless chuck from the bigger drills.

I need to chime in here because this 'dead horse' needs another beating:

I was always a T-Handle guy.  When the original C12 came out with its quasi pistol-grip style handle, I decided to buy it anyway.  I LOVED the drill, but didn't exactly love the handle style for all of the subtle "balance" reasons everyone has already mentioned.  Still, it was my only choice when I bought it, so It was an easy decision.

Later, when the T-15 came out I upgraded, thinking I was going to be in hog-heaven.  BUT instead began using the T handle only to realized the brilliance and efficiency of the C-style handle.

Fundamentally, what everyone has said is right:  It's absolutely a matter of personal preference, BUT NOT the way you might think. To use an analogy; most people may find upright bicycle handle bars to be the most comfortable, but only one style (the lower aerodynamic ones) will help you win the race.

Before having had experience with both tools, if you'd asked me I'd have easily told you I like the T handle best.  If I'd have drilled some sample holes in a store, I'd have said the same.  BUT, if you'd asked me to work with each for a week of varied tasks, I easily tell you I prefer the C handle. 

In my case, after moving BACK to the T-Handle I saw (by contrast) that the off-axis thrust of the T handle significantly increases cam-out when driving screws (unless you torque into it with your wrist) and it also increases attention needed when drilling to prevent lateral force on the bit/hole in situations.  The C Handle was  slightly less natural to pick up and hold but far more natural when holding it in position for work, and doing actual work.  For me, that easily trumps the 'better balance' of the T handle design.

I also found (to my great surprise) that the T-handle seems slightly more awkward in cramped spaces.   I thought it would surely be the opposite.  It does not surprise me that the nimble, efficient CSX is of the pistol-grip design.

Just my 2¢.

-Karl