Here's a list:
In X-ism,
You can't block mid air
You have 1000 power
All Supers have one variant costing full power
You can't taunt
You deal 1.10x damage
You have increased amount of Guard Meter
While in A-ism (Z-ism in Japan):
You can block mid air
You have 3000 power
You have three variants of most Supers and each variant Light, Medium and Hard cost 1000, 2000 and 3000 power respectively
You can taunt once per round
You deal 1x damage
You have the default amount of Guard Meter
And in V-ism:
You can still block mid air
Instead of having Supers, you have Custom Combo
You have 1000 power
You can taunt once per round
You deal 0.85x damage
You have a less amount of Guard Meter