well, one obvious difference is pack volume and since smaller rigs are way,way cooler this should be considered. i have a 27 cell and love the openings, some times i get a slow 90 turn on opening but most are on heading, never had line twists or chopped in 600 or so jumps on a Xaos27(dozens on a velo and 3 mals), but i've heard the 21 opens even better. next is the 27 is going to be a more rigid wing. not sure how much more performance you'll actually see from that. that's my .02
OK, I'm not super qualified to answer here but I have 250 jumps on my 21-98 at 2:1 and probably 25 on a 27-94. If I could do it again I'd get the 27, it's way more rigid, turns much faster, is trimmed flatter and flies faster. The 21 definitely opens sweeter though. All just my impressions from a few years ago, I haven't jumped my 21 for a couple of years as it's not really an ideal wingsuit canopy.
I decided a while back to use a conservative canopy for wingsuit, I jump a Sabre 120 or 107. Never had a problem with the Xaos-21 but could see it coming. Also I used to throw extremely low with the V-2/Sabre combo, the Xaos is a poor choice there. Since I don't throw low (skydiving) anymore I've considered using the Xaos again since it's so much fun to fly, I'll have to be much more current before I go there though. BTW I'm sure you're aware that most people consider a crossbraced canopy to be inappropriate for wingsuiting, great that you're having good results with the 21 and 27.