I use FL5 for drums since it's great at pattern-programming. I use Sonar to host all my VST-synths for the non-percussion.
I've got a lot of different synths/samplers for drums, but I always end up going back to FL5 to do my drums (hosted as DX in Sonar). However, I'm too attached to Sonar to leave it for the rest of the non-drums synths.
-Ido