There any significant difference between going with one over the other? Currently on a 70mm w/ a 4" coldside, and feel like this is the last bottle neck in the car.

Looking to go Wilson 105mm, but I can't seem to find a matching aluminum 105mm flange that could mate to it, 90mm would be my next best option (cheaper, easier to source a flange, and wouldn't need a throttle cable adapter).

Am I giving up any notable top end going with the 90 over a 105mm?

 

I definitely don't think so.

 

Without knowing your build specs it is hard to speculate if you are giving up any top end. If throttle response is at all a concern I would go with 90mm, although it sounds like it isn't