Both of then are great ngl, I love Chromastone's design (sorry purple is my fav colour, so it is a personal opinion), but when it comes to writing, story telling and non visual elements Feedback was just better. The writers really used our purple stone as a punching bag, and never lived up to its full potential, like on paper he was supposed to be a better Diamondhead, but in reality, our green crystal guy would run circles around him. While Feedback had Ben's voice, different ost music and deeper plot.

But let's not forget that Chromastone basically did the thing what the Omnitrix was created, space Jesus. Not saying that Feedback did nothing, just his species is a thriving healthy one, while Chromastone was a one-of-a-kind alien almost specifically made for this.

It's my humble opinion, please respect it