It's easier to crop an 18mm than to step back with a 21mm. Don't suppose you'd lose too many pixels that way. But the 21mm IQ is better than the 18 from what i've read and seen.

There's also the Sigma 20mm f/1.8, read Bryan's review, I think he absolutely loved it
...
*cough*

Zeiss also had 20mm in f/2.0, f/2.8, and f/4.0 flavours, mostly in C/Y mount (which you can adapt to EF for a few $). Some have better IQ and barrel-distortion than the others, google for reviews (nothing standard, just user reviews i've found). Most go for less than $300-400 on ebay, you could always buy one, try it out, and sell it for what you paid afterwards if you don't like it.