My brother has the Sigma 18-200mm and likes it for the versatility.
That being said, you'll be getting a "jack of all trades, master of none" lens with this range... same goes with either Canon or Sigma. Not sure what other lenses you have in your bag, but if this is the first non-kit glass... it's a good lens to learn, but you'll soon realize that your 50D can do so much more and will want to upgrade.
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So, I've got a Sigma f2.8/24-70. It's OK. My last two lens purchases were Canons, both L glass (17-40, and 28-105 IS).
I rarely use the Sigma any more (I got it when I was shooting a lot more film than I do now).
I ran some tests on the lenses - the Sigma really is middle of the pack. A little less sharp, a little more distortion than the L glass, but better than the Canon "kit-type" or "tourist" lenses.
If I need a 2.8 (and sometimes I do, especially with film....) it's OK. The IS on the 28-105 is worth about a stop, maybe a stop-and-a-half when shooting handheld. But I'm shooting digital on a 7D body, so I can effectively increase the film speed, meaning the difference between 2.8 and 4 is insignificant (costs a very minor amount of noise). For film, I'm shooting an EOS-3 body - my experience is that the negative effects of pushing film are greater than the negative effects from pushing the "speed" on the digital camera (esp. on color).
My vote goes for the Canon lens, unless you really can't afford them.