If you only download the occasional torrent, use Transmission. If your more of a heavy downloader, You'll be happier with Azureus - it's got more features that are useful for managing torrents. And it's interface doesnt get as cluttered up with 5+ torrents opened at once(or am I the only one that thinks that..?). Just keep java up to date. Oh, and Azureus 3.0 is the same as Azureus 2.5.0.4, just with a different GUI.
XTorrent is based on libtransmission, only with a new GUI. If you use XTorrent, you might as well use Transmission and save your money 😛
If you're on an old PowerPC mac, try using Bits on Wheels or Tomato Torrent. Though neither of those are unibin so using it on a newer Intel mac might be a bad idea.