Alright so just reading your post real quick I thought 24 105. But then someone pointed out you already have the 100 macro IS which is a great lens BTW. So as long as you're willing to switch lenses to get that 100mm range then you probably could go with the 17-55. I've rented both that and the Tamron 17-50 VC and non-VC versions. I ended up buying the non-vc model. The VC version was definitely less sharp than the Canon and non-VC. Some could argue the Canon is SLIGHTLY better than the non-VC but I think the biggest difference between these two probably comes down to the auto focus. The Tamron's isn't that great but it'll get the job done. I don't use this lens as much since going full frame but when I did use it, unless I wanted the 2.8 depth of field I'd stop it down 4.0 which sharpens it up a lot. If I was in your position, I'd go for the Tamron. It's a really nice piece of glass for the price. If you're thinking long term then the 24-105 is also excellent and obviously has IS and better focus system. I'd buy an ultra wide angle to make up the loss in wide angle shots. I bought the Sigma 10-20 for around $420 and have been really happy with it overall. Either lens will be MUCH higher quality than the 18-135.