I’d use drum patterns created with Beat Designer. I’ve set a Groove Agent One instrument track and I’ve inserted the midi device (with “jump” function activated).
I’ve defined some patterns and I’ve wrote a midi track to active them with note-on events. I’ve set a loop in which 2 patterns would sound alternatively. The behavior of the system is very strange when I press play: sometimes it doesn’t sound, sometimes it doesn’t change the pattern, sometimes it starts with the second pattern and after a loop it works correctly. 
Re-Drum of Reason is more stable, but I’d like to use Beat Designer for his velocity functions.
What is your experience about this?
I use C6.5 in 32 bit mode on Mac OsX Snow Lepoard.