what would be the difference in flying aerobatics on a pitts (i.e. conventional airplane) or on a pioneer 330 (which is ultralight) ?
Seriously, if you can ask that question, you're not ready to buy your own aerobatic aircraft yet because you don't know enough about them. You may end up buying something that's not suited to what you want to, or can, do.
What aerobatic flying have you done so far? In what types? Get yourself a couple of hours in things like a Decathalon, a Pitts, and the Pioneer (is that TRULY aerobatic?) to get a feel for what aerobatics in different kinds of airplanes is like. They'll all be so different that a discussion of them here will be fairly meaningless, I would have thought.
But be careful of one thing. As Budd Davison says "A Pitts will spoil you for other airplanes for life." And he's not kidding! Once you strap one on, anything else will seem tame and pointless! Oh, and by the way; a Pitts is NOT a "conventional" airplane!
Do some more research by flying more types. That's about the best research you can do. And it's the most fun, too!
Good luck with whatever you decide!