Opinions on best wide (budget) prime lens?

I'm currently in search of the best budget friendly prime lens in the 20-40mm range. I currently own the 50mm F/1.8 and I love it, but it's a little hard to work with inside on my T3i as I seem to always be backing into walls trying to get good shots.

I'm kind of stuck between three lenses at the moment, the Canon EF 35mm f/2, the Canon 40mm EF f/2.8, and the Sigma 30mm f/1.4 EX DC HSM.

I was pretty set on the 35mm until I started reading about the 40mm which sounds like an outstanding lens, but I wasn't sure if 10mm is much of a difference from my 50 and worth having two primes so close together. Can anyone give an opinion on the IQ difference between the 35 and 40?

Additionally, I've read that the 30mm Sigma is THE perfect prime lense for the APS-C sensors. It's a little bit more than my budget allows, but if it's substantially better than the other two lens's I'm looking at, I'll save the extra cash to make that purchase.

Basically, just looking for thoughts and opinions on these three lenses and what everyone else thinks is the best prime to go w/ my T3i. Or if there is a better lens in the same price range as these please let me know!

Thanks,
C