Haven't used any torrentclients for a year now, but I think µTorrent is the best client. It's VERY light, doesn't eat performance, like, at all. And it provides all ye need.
And what I think about Azureus, well haven't even tried but I saw my friend using it. Its' c00l, but HEAVY. I'm not sure but at least it was java-based-client, which makes it so heavy.
And for linux, don't even think about using client with GUI, it is just so much lighter without it. BitTornado is good client for linux, you can use it in "textonly"-mode and with GUI, if the textbased feels too hard to use somehow..