BitTorrent bought µTorrent some time ago and since then BitTorrent is µTorrent except it has the different name. So the two clients are identical. BitTorrent does not even maintain a forum anymore and refers all users to the µTorrent forums.
However, BitTorrent adds a program called DNA to its download. DNA does nothing to help torrents and is always running, even when BitTorrent is closed. DNA can be deleted with no affect on torrents, but it is better to get µTorrent and not bother.
Both are equally fast and both are among the fastest bittorrent clients.
See the related link for more information.
You can get an invite to just about any torrent site here: http://www.torrentauction.com