I went into Harmon Photo which is a specialty photography place by me and I played with these two lenses. I was back and forth and back and forth so much I got confused and one of them focused soooo much faster because it had something extra or different I want to say it was the 28-135. Does anyone have any advice? I want a zoom lense my budget is about $400 and the Sigma is cheaper but I'm scared of it being a worse lens. Help!