ok none of the bittorrent clients you use have any effect w.r.t speeds of your downloads. That is totally dude factors which are not in the client control.
1) Your available bandwidth - If your on a 256kbps the max download speeds you can ever get is 32KB (256/8 do the math).
2) Like DrGrudge mentioned some server restrict your download speeds. Depending on your upload speed and so on.
Client features one should look at is part file downloading, seeding capabilities, debuggers, file details, file priorities and so on. Its like asking which download manager is the fastest. Given a T1 line and 2GB file all of them will download it in more or less the same time. It all depends on the added features and easy of use.