Which lens peoples experience etc
Concerning the sigma 10-20, the f4-5.6 is sharper than the f3.5.
As far as I'm concerned, the Canon 100-400 is a wonderful lens if you can cope with the pump action zoom. It is a tad sharper than the 120-400 from sigma as far as slrgear.com is concerned, but it's also pricier.
I would really recommend going to your local dealer with your body and a memory card and trying them in real life, then look at the results at home and see if they please you.
If it was me, I would have the 10-20 f4-5.6 (I already have it in fact ) and the canon 100-400 (I have rented it a few times and it's a charm).
Renting the lenses you're interested in would also be a good way to know if you'll like it.