I suppose it depends. B20 will net you the most power but has higher compression and won't stand up to boost as well as a B18 will.
I was exactly in your shoes once. I decided on the B18 route since my research had told me they are best when it comes to holding boost, at least stock for stock. I ended up going B18B non-vtec, adding turbo a year later. If it weren't for body rust, I bet the motor would still be around and kicking today.
There really isn't a right or wrong answer here, there really isn't a "best" motor to get. I suggest you look into each motor and determine what suits your needs best.
I refer many people to
*this link* too: It talks about the different motors and pros/cons of each when it comes to forced induction. It's also a bible when it comes to learning forced inductions systems on a Honda.
