The Q76 is a little bit bigger over a cm wider and in real life a few liters more too. It has a slightly faster rocker and a different bottom shape and is also a bit more compact. But tests show it still has a lot of the EVO feel and is "in some ways even better" (as one test put it). I have q76 convertible myself but I like it so much as a quad I haven't even tried it as a single. But apart from the extra weight of the boxes, one can sort of say the quad 76 shape is a "more modern EVO". And, you do get an amazing quad fin board at the same time which you very much may prefer. So I say you should go for it unless the E75 was a bit on the big side already. The the Q76 will be even bigger.
Fin: The best size will depend a bit on what sails you are using and how heavy and back foot heavy you are. But I would for sure try to get a hold of a Drake Natural (from EVO or small Kode, your supplier might have stock). I figure a 23 would be a good all round size.
Note that it will also be possible to sail the Q76 as a tri fin board which can be nice for some particular purposes.