I bought SGP Ultra Thin Air (clear type) case.... I'm very happy with it.... it made of Polycabonate which is very hard and strong. Yes it do have some tiny scratches but it's very hard to see because my phone is Silver. I's almost 5 months now and it's still crystal clear no yellow.... I do believe Polycabonate will not turn yellow overtime.
But.... SGP case do have some problem with QC.... I suggest you bought it in the shop when you open it up and see the QC problem you can try the others... not to waste time sending it back and forth with online store.