ultra wide angle zoom for canon 60D

I am going to europe for a 3 week trip. I have a canon 18-200 zoom but I am looking for something a bit better in the wide angle end of things. i was looking to spend in the $400-$700 range. What are the thoughts? Canon vs sigma vs tokina vs tamron? How LOW should I go? 12, 11 or 10 mm. There are a lot of lens out there. The canon 10-20mm, the sigma 10-22mm , Tamron 10-24 mm What is the opinion of the users out there?

The other options are the canon 15-85mm and forget the low end. or keep what I got
Thanks!!!!