I dont think there is any difference, the ex721 is just the newer version. I have both rims in my garage and i cant see a difference between them, both bloody strong rims.
why did you not take the bb out first, before smashing the hell out of it, or is the bb broken? But i know how much fun it is, ive done it with an old shitty frame a while back and i destroyed it, was so fun.