The basic filters sound almost the same - clean digital EQ. ProQ is enhanced so high-end boosts don't get pinched up against the Nyquist frequency (half your sampling rate)... so ProQ behaves a bit more like analog EQs in that regard.

...And then ProQ has loads of additional features.

I've got Pro Q 2... the big factor for me is the interface, Fabfilter UI is really good. Also... I prefer Pro Q for mid-side (although I bet I could learn how to do the same thing in EQ8).

Fabfilter know their own worth though, they have a policy of not doing crazy "80% off" sales... so at the moment my strategy is use EQ8 where I can, use ProQ2 for mid-side and really surgical stuff, where I know the interface better, and use TDR Nova for dynamic EQ so I don't need to drop $$$ on upgrading.

You don't need it make good music. If you like you can just chain multiple EQ8s back-to-back to get 24 filters or to do a combination of L-R and mid-side EQing.