I don't think there's much difference between x11 and other x-algos. I believe they all use same algos just different order and rounds of hashing. Prime I just know that it looks for prime numbers, and I think the only GPU miner is a closed source claymore one that forces you to "donate" to him. Neoscrypt seems like a pretty good option for GPU.
Honestly, it's a tough decision, but I think as long as you stick with relatively popular algos (that have well developed and ideally open source miners) then you can't go too far astray.
not sure about the rrest of your thoughts.. but if x11 is similar to x15 etc... wouldnt we just go x17 or whatevers the highest?
and gpu isnt that Blake? unless i missed a change there..which is possible.
if that happened, i would jump off blake and go... nscypt or ..again only cuz i have done it.
g