I have the 11-22 but I've tested it against the 10-18 a few times.
11-22 is sharper, but not that much. The 10-18 has CA in the corners and sides, but lens profile corrects them very well, so in most cases you won't see much difference between both lenses. If I showed you two images with corrections applied, you won't be able to distinguish them.
The 11-22 has better contrast and flare resistance - this is importatn, as I like to shoot against the sun.
The 10-18 has better lower distortion. At 10 mm it has a lot, but at 12-14 mm it's better than the 11-22
They both focus fast, are small (the SL1 + 10-18 is not so much bigger than EOS-M + 11-22)
There is one clear advantage of the 10-18 - it will work on DSLR's with real VF and you won't have to worry when canon will release a decent M body.
The 10-18 is almost as good as the Sony 10-18 F4 OSS, which costs three times the canon. If I were thinking in perspective, I'd get the 10-18, because M's future is not very bright with the flop M3 is going to be and the fact, that canon are not devoted to the M line.
If I knew canon was going to release a 10-18 back when I bought the 11-22 I wuld have tohught about buying it twice. At the time it was the best (and still is) canon UWA for crop bodies.