Think this will give you a easy guide if $$ is not an issue:
Buy the sigma if you use mostly from 70 to 200mm and would like taking indoor once in a while.
1. F4 on the wider end is really useful
2. Sharpness is comparable from 70 to 180mm to the L
Buy the L if you use mostly from 200 to 300mm and would only take outdoor (zoo etc).
1. F5.6 is painful
2. Most of the time a tripod is needed
3. Push-pull is troublesome
4. Sharpness throughout the range (think this is the only reason it will win over many people).
Have gone through this and I select the sigma as I seldom use the tele end