Hi, I currently have a Rebel XT with a canon 28-80 f/3.5-5.6 lens and I mainly shoot pictures of mountain biking, there is sometimes that i shoot in low light as well. My question is if i should get the canon 70-200 f/4L lens
http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&fcategoryid=150&modelid=7345
or the canon 70-300 f/4-5.6 IS lens
http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&fcategoryid=150&modelid=11922

What I would like to ask you guys, is if the IS on the 70-300 lens is enough to make it a nicer lens than the 70-200. I have heard a lot of good things about IS, but I have also heard a lot of good things about canon's L series. Or maybe there is a different lens you guys recommend that is in the $500-$600 range? Thanks

Yes the L glass is better than the standard but also the 75-300mm is not a bad lens and has the resolution to cope with the Rebel XT. However I'm not so sure that f/5.6 is a good thing for your lowlight shooting. Don't forget that while IS stops camera shake it does not freeze the action your shutter speeds at f/5.6 in low light will be significantly slow.

Why not save a little more and get the new 70-200 f/4 L IS? A cracking lens and not as expensive as the 70-200mm f/2.8 L IS by a long mile!

Another route would be the Sigma 70-200mm f/2.8 on a monopod. This is a great lens very sharp and lovely contrast. Color is a bit warm but can be easily corrected post process or with custom white balance. It's a massive bargin lens and very nearly L class. It's only flaw is hunting focus thats slower than L lenses you mention. It's totaly usable though.

How's your panning technique by the way?