I want to finally upgrade my kit lens to something with a fixed aperture and wanted to get others imput on what to buy. I use a Sony a200 and am looking at the Tamron 17-50mm f2.8 or the Sigma 17-50 f2.8. I have read lots of good things about the Tamron and it is about $400 but I also have a sigma 70-210mm f2.8 and really like the look and feel of the Sigma. But the Sigma lens retails for about $700. I plan on using this professional for portraits and wedding photography as well as architechturual photography. So should I be cheap and go with the Tamron or is there a benefit to spending the extra $300 and getting the Sigma instead.Your thoughts?