I like the BK2 better because it is easier to tune for me than a B4. I had both and I ran them back to back and after getting them both setup to the best of my ability, I was faster than with my B4.
There are things that I don't like about the Bk2, but they are minor things.
The one thing I want to get my losi to do that my B4 did, was rotate more in the corners...