For me, it came down to the tread pattern. I chose 285/40/18's because I wanted the BFG KDW NT's. If they made it in a 295/40 or 45/18, I would have chosen it to eat up more the space in the fenders.
The one thing I really hate about our cars though is the size of the fender opening in the front. Once you lower the car on 18's, the fender looks like it is swallowing the damn tire and wheel combo.
I have 255/45/18s up front by the way.
285/40/18's in the rear.