uTorrent has recently been updated to version 1.0 (Windows users are up to version 2.0.2), its first major point release since going beta on Mac OS X. After living most of its life in Windows, the popular BitTorrent client went beta on the Mac in late 2008, and has seen a host of updates since then.
This might be a time to consider trying uTorrent. In my tests, the app launches faster than Transmission, and occupies a smaller footprint -- its DMG and the app are both smaller than Transmission. Whether or not you end up switching to uTorrent, however, is another matter, as Transmission is comparable in its feature set. This really boils down to personal preference.
While BitTorrent apps live, and do whatever they want, freely on Mac OS X, the story is different on the iPhone. Apple has kept a tight lid on restricting BitTorrent-related apps (such as a BitTorrent client controller) on the iPhone, noting that "this category of applications is often used for the purpose of infringing third party rights."
uTorrent 1.0 is available as a free download at the uTorrent site.