I take it you want an ultra-wide lens for landscape stuff? Sigma 10-20mm seems a popular choice because it's one of the cheapest out there, much cheaper than the Canon version. There are two versions of the Sigma, an f
/4-5.6 version, and a
constant f/3.5 version - that latter is newer and more expensive. Think the variable aperture version is about £360 and the f/3.5 version priced at over £400. Used you'll get the former for closer to £300 probably.
Tokina does a 12-24mm f/4 version, which is arguably the sharpest UWA out there for the money; my version was on a par with my Nikon 12-24mm f/4, which was tack sharp. I picked up a used version for £250 - I think new they go for about £440.
Tokina also does an 11-16mm f/2.8 and a 10-17mm - regarded as very good but not sure on the prices; look on
www.camerapricebuster.com to see what they go for.
My advice if these are the kinds of lenses you're after; don't think that because a lens isn't as wide it won't be any good.... 2mm at the wide end is a step backwards in realworld terms so don't just think that it has to be 10mm or nothing. From a personal POV, I didn't like the variable aperture version of the Sigma - it had wierd distortion that wasn't confined to the edges on both copies we had at work. Over a DPS the images looked awful, but a lot of people like this lens. The Tokina 12-24 is amazingly good value, especially if you get a used one, and the 11-16 is supposed to be the nuts for sharpness too.
As for the grip, the 3rd party grips are actually pretty good these days