You can argue about the topic til death if you want to...
Everyone who still uses BitSpirit, BitLord and god knows what should upgrade switch to either uTorrent or Azureus. Linux/BSD users should consider Azureus or rtorrent and OSX has a few to choose from where Azureus and Xtorrent seems to be among the better ones. Most other clients (I may have missed one or two) are highly outdatded, abuses the tracker itself and/or sends corrupted data. Many clients also dont support encryption which is highly recommended for speed reasons. For those saying that client X is faster than client Y I'd say that is pretty much bullshit, it all depends on what peers/seeders you connect to and how aggressive the client is towards others. In general Azureus, uTorrent and rTorrent are pretty fair to each other. Azureus, uTorrent and rTorrent may also provide a little speed boost due to support for encryption but it taxes the CPU slightly more. Also keep in mind that both Azureus and uTorrent supports DHT (rTorrent doesnt) which is really nice on public torrents in case the tracker goes down. Keep in mind that Azureus and uTorrent uses different algorithms meaning that they wont find each other, you can though thru another Azureus/uTorrent user depending on what client you use. This feature is although requires a bit of pressing power on a router which most routers for home use lack resulting in a dead router after a couple of minutes/hours or a deadly slow one so I'd suggest that you disable it. A few routers can handle it though perfectly like Linksys WRT54GS (V1-V2 mainly) and Asus WL-500G Deluxe/Premium-series although you may need a 3rd party firmware. Also use reasonable setting, connecting to 1000 peers wont make things go faster on a 4mb connection or whatever you have. Roughly I'd say 80 global connections per mbit (and 80 max per torrent) and dont go over 250 if you have a router even if you have a faster connection than 4Mb, it will most likely slow it down to turtle speed and eventually hang. As for upload slots I'd say that you should have roughly 4 per torrent if you have less than 5 mbit otherwise bump it up to the double.
Regarding the clients themselves Azureus uses in general more memory than uTorrent although feature-wise its much better and it actually does have a disk cache that works and they use about the same amount of CPU. General misconeption regarding Azureus is that its slow, mostly because people have an old Java Run-Time installed and doesnt read the docs. FWIW I have an old Athlon 500Mhz computer that runs Azureus among a lot of other apllications perfectly fine. uTorrent doesnt on the other hand use Java at all, it free and all but keep in mind that its closed source and Bittorrent Inc which works with MPAA and RIAA have bought it meaning that you may get some rather nasty surprises later on. No "ET phone home"-issues have yet been reported though. rTorrent is an excellent (text based) client for Linux/BSD which has little memory footprint and is also like Azureus open source, only downside is that it doesnt support DHT yet.

And btw, bandwidth issues are either caused by your router and/or by improperly configured clients. If you router supports QoS (the ones mention above does with 3rd party firmware) use it and configure it properly and you'll be fine. Otherwise set upload to roughly max upload speed to 80% of your total upload. Be aware though that if you have multiple computers you'll need a router supporting QoS if you want it to work smoothly.

//Danne