I'd appreciate any assistance on determining the differences between the Sigma 10-20mm F3.5 EX DC HSM or the Sigma 10-20mm F4-5.6 EX DC HSM. I understand the aperture difference and the advantages to the fixed 3.5. My question is more along the lines of build quality and glass quality. The price difference is $170 and I"m wondering if the 3.5 is a better lens in any ways other than being able to shoot in lower light and/or at a higher speed. I have a Nikon D5100, btw.