the question is whether you like such a wide focal length, or not so much
even if the tokina is rectilinear and with basically no barrel or pinchusion distortion, the perspective of an 11mm is not something people will feel natural
The 10-22 on an APS-C crop body is an absolute gem, and is a lens you'll use a lot on your trip.
15mm sounds wide, but do the sum; 15 X 1.6 = 24. Hmmm not so wide.
The 10-22 is a hero lens in the EF-S lineup and would be very quick to sell on Craigslist if you transition to FF at some time in the future.
the tokina 11-16 f/2.8 is an extremely nice lens; if you're going for a lens of that focal length for APS-C, and have the money for the tokina, it's basically flawless
I have to disagree with the "flawless" adjective. The Tokina has considerable more chromatic aberration than the Canon. I tried them both before settling on the Canon. I'm not saying the 10-22 is without any CA, but it is less than the 11-16. Lightroom has a lens profile for the Canon lens and so removing CA involves a single mouse click. There is no lens profile for the Tokina in LR so removing its CA is a pain in the patutti!
Diane