Another vote for 5x7" here. My 4x5" has hardly been used since I got 5x7", and 8x10" is seriously bigger.
I've got a 8x10" for "rebuilding", but I doubt it will see as mych use as the 5x7" when I'm finished with it.
Another plus with 5x7": Many 5x7" enlargers will fit in a "normal" house - 8x10" enlargers are a different matter. 5x7" can be contact printed, or easily enlarged.
I buy most of my film - all sizes - online. But my local pro shop can get me anything I want: 9x12cm, 4x5", 5x7" and 13x18cm, as well as 8x10".