I owned them both and the BVision is a bit faster. If you can't get hold of a BVision then you wont notice a difference in speed during usage, it's only really when you start software benchmarking and can view concrete figures.
If you have a Mediator then I would say the best card to get is a Voodoo 3 (in particular, the 3000 model) as the 3000's drivers are more mature.
