Vuze is pretty slick these days. Its size is only 'cause its a "media center" with a player and a bunch of front-page crap like YouTube channels, which I never use anyway. Using it just for torrents, I don't even know its running.
But then again, I'm partial to it because I've been using it for so long, I know what all the controls do (or, more accurately, which ones matter). If I had to start out nooby, I donno which I'd use. I guess I'd go with Joel's suggestion because he's smarter at the wiz-kid stuff than I. If its lighter, its probably more efficient, theres no arguing that.
I think with modern comps, its all about user interface. After all, these things are well designed by professional geeks. Its not like any of 'em are outrageously flawed. All they do is assemble torrents. You don't actually have to know how BT works, you just have to know how to operate your client. Don't expect something to download if theres no seeds, that sort of thing.
Size and RAM of these things have been bragging rights between the competitors for years. Then people turn around and keep a FPS game running, minimized for a week. Or like me, a trading platform, which has constant data streaming through it, and is encrypted, and a megaton of GUI shit slapped all over it.