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.