Comparing the ultra-wide, crop-sensor camera lenses is an extremely difficult task, so I’ll put the summary first: they all deliver excellent image quality, and you can’t go wrong with any of them. To my ‘just taking pictures’ assessment, they are all excellent. There are some differences though, which I’ll try to point out so that you have a better chance at choosing the one that’s best for you.
Sigma 8-16 f/4.5-5.6 is the widest (and remember, 8mm is 20% wider than 10mm, so it’s a very real difference). Not quite as sharp in the corners as the others, and lower maximum aperture, but it’s really pretty good, especially considering it’s the widest of the wide.
The Sony 11-18mm is a decent lens, although it really isn’t sharp wide open. It needs to be stopped down a stop to give you the images most of us want. It’s nice and small, which is important when you’re only carrying an ultra-wide in the bag “just in case” you need it. Overall, a decent, but not a great performer.
The Sigma 10-20mm f/4-5.6 has a bit larger range and provides excellent image quality, although it has a bit of barrel distortion at the wide end, fits no worse than the Sony. It’s quite sharp in the center even wide open, a bit less so in the edges until f/5.6 or f/8. It’s internally focusing, so if you plan on using a circular polarizer (not always the best idea on an ultra-wide), you might prefer this one. Plus, it’s a good price.

But, like I said above: they’re all excellent. We hardly ever get anything but happy comments about any of them.

Try and Buy Program: Lensrentals is pleased that our friends at SuperdigitalCity have made a special offer for our customers renting this item and considering purchase. Simply email your Lensrentals Invoice number to SuperdigitalCity and they will email you back a coupon code good for $21 off of the Sigma 10-20 f/4-5.6 lens.