zoom lens advice

[faffer]

I picked up an old eos 20d which is great (i'm just a beginner having a play around). It's got a crappy kit lens which will do for now, but i'd like to play around with a zoom. I don't want to spend a fortune, but it looks like i'll have to spend a little bit to get something half decent. I would quite like a 100-300mm but everything cheap looks rubbish, and everything good is incredibly expensive (but if anyone can recommend something please do!).

It's not just for one specific thing, but one of it's jobs will be photographing rowing races, not as demanding as speedway or something but still just about 'action'! On the face of it having the extra f-stops would be handy as obviously weather can be overcast, but how necessary do people find it in reality?

So i'm left looking at the Canon 70-200 ef L which looks great, but I also noticed the Sigma EX 70-200mm F2.8. Obviously the bigger aperture would be nice but what is people's experience of these lenses? I haven't found much bad written about the Canon, and it's down to around 4-500quid at the moment (top end of what I want to pay really), looks like I might be able to pick the Sigma up for 500ish.

Any advice much appreciated.