My experience with the 3rd party wide stuff for Canon FD is: don't bother. The difference in image quality is simply not worth the cost savings. And Canon FD stuff is relatively cheap these days.
I'm not a big fan of the 17mm. That small max aperture is annoying, and to my eye the lens vignettes a bit. The 15mm is a great lens, if you can work with the barrel distortion. The 14mm is just downright weird in its effects, that lens is hard to use well (and is still really really expensive because it's so rare.) So yeah, the 20mm is a nice choice.
When I was testing all my lenses on one of those spinning ground glass video camera adapters I put together a big chart which might help you in your quest: