We got a bunch of CFR Glow FBs for our tournament last weekend and next, and to my surprise none of them were flat like 11x FBs. They aren't as domey as most recent productions runs, but they definitely were not flat topped.
In the roughly 10 years I've played DG I've carried a CE/Proline/Champ FB in my bag for roughly 75% of the rounds I've played(they were all FLAT topped). The remaining rounds I've carried the Pred or XXX. Currently I carry a 176 Opto XXX. My FB muscle memory still causes me to make occasional mistakes with the XXX, so I can say that they are different. There are subtle differences in speed and flightpath due to the different shape/size of the wings (FB has notch, XXX is more like FL, XXX has wider wing) and difference in diameter (XXX is larger). Here's how I'd break it down:
FB advantages--tighter flight path, less speed sensitive, fade is more consistent across release speeds and wind conditions
XXX advantages--faster, slightly more glide at high speeds, turns a bit slower and this makes it a bit more versatile on lines, easier to get a bit more D than FB
Basically a good flat FB is a bit more predictable, and probably better for power players. It is a bit slower, but comes out of a deeper anhyzer release, and fades earlier at high speeds. The XXX seems to take a bit less effort for me to throw than the FB. It has a straight then dive flight more than the FB which fades more laterally. With enough power or into a headwind the XXX will hold straight longer than the FB and fade later. I find I can smooth it with any amount of anny release at distances of 250'-300' and get a perfect reverse flight, but if I really punch it with too much anny out further than that it often pulls out late and ends up pretty straight. The FB would still pull out and go left on these shots.
Overall I find I like the advantages of the XXX enough to learn the differences over the FB. This combined with the de-evolution of the FB's profile and difficulty in finding flat ones make the XXX a better choice currently.