Aftershock has smexy air mixup jumpy low things, crystal has armor armor and better armor launchy smashy rocks. Crystal also has sound fx in the game.
Ok, I'm a newb, but I see it as crystal being better vs rush-down and Aftershock being better the rest of the time.
Aftershock anti-zones quite well (it seems to me) and you can really build nice mindgames off his jumpy low smashing silliness.
Crystal, with armor and the armored, faster launcher thing is nice. If you get momentum on your side it can be tough for the opponent to get it back, dealing with oh rocks, armor and the threat of your launcher with meter.
Just a newbs take on things, a more experienced player might shit on my opinion. But that's how I see the variations.