Hi Matt and welcome to WorldPhotograhyForum
I don't have any personal experience of either of these lenses but have also read good things about the 17-70 though have heard mixed reports of the 18-125. Whether the extra reach would be worth a slight drop in IQ is something only you can answer. For me I'd take quality over reach everytime, but if you
need the reach to get your shots then this thinking doesn't work.
Another lens to look at would be the Canon 55-250 IS, it will compliment your kit lens (do you need to buy a lens that covers the range you already have). The 55-250 is getting a lot of very good reviews, it comes in at a similr price to the other lenses and would offer you even more reach -
http://www.camerapricebuster.com/Can...5.6_IS_pc.html