I have tested over 20 bittorrent programs, including those two, and they all run at the same speeds when properly set up.
Features and support is what I think are most important.
Vuze is one of the best due to the many features available within it and through plugins. However, it uses more RAM than other bittorrent clients. It has excellent support too.
BitTorrent bought uTorrent some time ago and since then BitTorrent is identical to uTorrent, except that BitTorrent adds the unnecessary program DNA to its download. DNA does nothing for the average user and is always running, even when BitTorrent is closed.
For that reason uTorrent is a better choice than BitTorrent.
uTorrent is small, uses very little RAM and has all the basic features needed. Also has excellent support.
uTorrent and Vuze are the two best IMO.
Best Free BitTorrent Client
http://www.techsupportalert.com/best-free-bittorrent-client.htm