The Tamron is 10-24 which gives you a bit more flexibility. I used to have the Tamron when I had my Nikon kit. It was very good optically (if not brilliant) but on the D40 it was a total pain to focus while on the D7000 it was much better. It will probably be ok on your 60D.
I now have the Sigma (not the 3.5 one) for my Pentax. Focus is ok but I am not extremely happy with the IQ, lacks a bit of bite I think.
I'd try out other ones as well like the 11-16 from Tokina, the 10-22 from Canon and the 8-16 Sigma.
By far the best ultra wide in terms of IQ and AF speed I have had was the Nikon 12-24 but those 2mm on the wide end make a huge difference.
Thanks guys for the replies and I have just got myself a Sigma 10-20..

3.5 or 4 (and why)? I've got the f4-5.6 version (but iirc, the price difference was a lot more than £80 when I got mine).
I just got the sigma f4 recently, great lens, you won't be disappointed. I just had it out for its first proper runlast night, just what I wanted. Went for the F4 as I was mainly using for landscape and long exposure so the extra for the 3.5 wouldn't make much difference to me.
Barvas Ruin by M+M Morrison, on Flickr
I did get three lens which I got from Gibralta yesterday..
Any chance of some links to the shops/product pages with the costs. Im looking to buy the sigma 10-20. Also looking for best prices for a samyang fish eye and a nifty fifty ( whatever that is lol. Sounds like it will be £50 )
Thanks for your time.