Older wide/superwide options for 5dmk2?

Thanks for the info and advice! I finally got the Tamron 20-40 out and tested it... and here's my take...

Build: The Tamron is a bit larger, heavier and uses more metal - it feels sturdy and looks quite a bit better in person. The Tokina is much lighter and is just about all plastic, but still has pretty good build quality.

Focusing: I will say the Tamron is LOUD, but I got used to it after a few shots. The Tokina is quieter (though still somewhat loud), and seems to hunt a little bit less than the Tamron. I'd probably give a slight edge to the Tokina.

Image Quality: Here's where it gets interesting. I set up a tripod and took similar images with each - the Tamron at 20mm f11 and the Tokina at 19mm and f11 (I know the purists will complain, but since I'd use each of them at their widest it didn't seem very practical to keep them both at 20mm). The first thing that jumped out was the color rendition. The Tamron gives deep, saturated colors that are very accurate.  The Tokina is a little washed out - not as saturated and blows out more highlights.

When I first looked at sharpness (at 19mm and 20mm), I was surprised by the Tokina. Comparing the two, image centers are almost equally sharp, with maybe a slight edge to the Tamron (I actually favored the Tokina in one shot). Move away from center and you see the differences more clearly. The Tamron is surprisingly sharp out towards the edges, where the Tokina starts falling off quickly once you move away from center. Where the Tokina turns foliage into a smear in the corners, the Tamron holds strong and provides good definition. Not sure how to explain it, but the Tamron also has a very pleasant way of rendering edges compared to the Tokina... my guess would be that it can resolve more detail due to better quality glass/coatings. In terms of C.A., however, the Tamron may actually be a little worse. Purples and greens are abundant in the corners of each, with a little more in the Tamron samples.

I also noticed a difference in distortion between the two. The Tamron shows some barrel distortion at 20mm, but it's very "normal" looking and easy to correct. The Tokina shows some as well, but also adds some odd mustache distortion to horizontals - not a big deal for landscapes but architectural shots are a little tougher to correct.

Just for fun, and since I had it in my shirt pocket, I also tested the Tamron zoomed to 40mm against my ef 40mm f2.8 (at f8) and was surprised that the Tamron held its own in terms of sharpness and clarity. I'd still give the pancake the edge, but it's not as different as you'd think.

Overall, I think the Tamron will stay in the bag and I'll part with the Tokina. It was well worth the price I paid for it, and fun to use, but it's time to upgrade. I can live with noisy AF in exchange for better image quality. I wish I had the canon 20-35 to compare it to - but I have used the Sigma 10-20 on APS-C and an Olympus 9-18 for M43, and the Tamron compares favorably, I think.