ANEWUSER said:
Thanks Matt, but personally what do you recommend for between the two that you mentioned. BTW know of anyway that sell them at a reasonable price.
Hmm, I'll assume that you're using the Nikon System since your question refers to the Nikkor 20mm and Sigma 20mm that I mentioned.
About the Nikkor, it is very small and very light, you won't even know it's there (sounds like a tampon ad), but will serve as a good walk-about lens. It has very decent images wide-opened and is very sharp from 5.6 onwards (sweet spot at 8). Colors aren't as warm as the Sigma's and it does suffer from more distortion than the Sigma, however, it has less of a problem with flare.
The Sigma is very much heavier and very much bigger (about 3x more), but is sharp from 2.8 (images are soft at the edges at 1.8 so I tend to avoid using it below 2.8). It's colors are warmer than the Nikkor's and IMHO I find it to give better contrast too (I found this to be so with the 105mm macro lenses as well). With almost no distortions this is a good lens for shooting buildings. My major gripe is its problem with flare.
In terms of pricing, the Nikkor is about 1.5x more expensive but that wasn't a factor in my going with the Sigma. I spent a month comparing images taken by others and myself (borrowed the lenses from friends) and found that I was leaning more towards those taken with the Sigma.
As to where you can get them at reasonable prices, you might want to check with J3:16, MS and CP (yes, CP has started selling Sigma again). Their prices vary by only a few dollars so go with the one that gives you the best service.
2nd hand? I rarely see it being put up for sale.
Cheers,
Matt