Xtorrent is free while in beta, but like all other Dave Watanabe apps, they'll cost eventually.
As for BitRocket vs Transmission, etc, I currently use BitRocket. But I only use BitRocket because, in its current state, Transmission is banned from OiNK.
I had high hopes for BitRocket when it first came out, but there hasn't been an update for it in almost 6 months. If the tracker you're using hasn't banned Transmission, I'd recommend using it: it's lightweight, fast, and very OSeXy.
If you need the "has everything you could ever want and plugins for the things you didn't know you wanted" solution, Azureus is about your only option, though.