After reading the reviews hop on over to www.the-digital-picture.com to check out the "ISO 12233 Chart 100% Crop Comparison." This tool is really cool because it lets you can compare two different lenses at different f-stops and zooms (mouse over the chart to see the difference.)
If you're still up in the air.. rent the lens(es) from http://borrowlenses.com/ to test it out first hand. Renting is a good alternative to dropping $1,000+ on a lens that you may only use once or twice a year. :-)