howdy soul, we've got a couple threads here on the very subject.
Let's just say they are both good products with lots of good points and a few bad ones... both scale well on properly-configured servers. UBB's achilles heel is its search feature and heavy I/O (improved in 6.1.0 final), vB's achille's heel is that a large number of concurrent users can kill the database in a heartbeat. Counterstrike found that out.
Good news is that 6.1.0 will be released soon and fix a few of the major problems persisting in 6.05
