Here are a few suggestions to think about;
http://www.the-digital-picture.com/R...ns-Review.aspx
http://www.the-digital-picture.com/R...ns-Review.aspx
You can compare the lenses on the ISO charts on this site;
http://www.the-digital-picture.com/R...mp=4&APIComp=1
When comparing it is best if you keep the apertures the same and the focal length the same. As you stop down an aperture (make the number larger) lenses tend to get sharper as they move toward f/8 (a generalization that is close to accurate, a disclaimer put in for those that would point out such and such lens sweet spot is at f/5.6)