If you want to make 303 type sounds, you will probably prefer the 8580 chip. You may like to put two of them in your c64 as well. It is possible to put an 8580 chip in a 6581 computer, but it takes a little more work.
You should also have a look at the midibox sid project at midibox.org (follow the micro C apps link), if you're interested in SID chip instruments. It takes more work again, but it is capable of much more than the prophet64. For 303ish sounds, you can have a look at the bassline mode. You can also find info and links regarding the SID chip revisions.