I tested the current Mac clients of µTorrent versus Transmission using the .iso file for the latest version of Ubuntu. Both clients were fast, reaching the maximum of the 1.2 MB/sec connection, but while Transmission’s download speed varied wildly between 300k and the 1.2 MB/sec cap (and Activity Monitor bore the variance out), µTorrent hit the 1.2 MB/sec mark early and maintained it all thoroughout the transfer. A second test narrowed the gap, but the edge still went to µTorrent.
This has been my experience using uTorrent on Windows also; µTorrent has always been a faster client than any other GUI torrent client out there. Transmission might have the edge with the elegant, tiny UI and the lovely web interface, but if you want the data fast, use µTorrent.
But this was just my highly unscientific test. What’s been your experience?