BitComet is a hated application by other client users... I haven't really understood why, but it's something about it making cushion files.
uTorrent is probably the tiniest client there is. Use if you have... well... having little space doesn't make space, since torrents are for rather large files.
I use a variation of Azureus,
BitTyrant. It's 18MB but very well worth it
Go check it out and you'll see why.
I love Azureus's "aerial" view of the swarm. So does my sister. We watch it for hours XD