Change of client will not affect your download rate in any way possible........ unless one of your clients was configured badly and you don't allow for some bandwidth slack. Or if you've put the incorrect income ports online instead of anything between 6881 to 6889. Or if the client just can't get past your firewall or router for some reason.

If there is a difference, it is NOT because of the client that you're using, but because you have misconfigured it, or haven't properly prepared your computer for the network. The reason the clients make no difference is because every torrent runs off a 'tracker'. It's these trackers that determine your download speed, which is kept on an external server. It's nothing to do with the client at all.