Hi everybody; I want to buy one of the following lenses to use on my Pentax K10D; 1. Sigma 24-70 F/2.8 EX DG Macro 2. Tamron SP AF 28-75mm F/2.8 XR Di LD I've searched both of them and I am sure both are good enough for me. Generally it is said that Tamron was slightly better optically but built quality of Sigma was better. I will use it as a walk around lens. Is there anyone used both or just can give an advice about it?