Hello All

I am looking at purchasing a new lens and have an upper limit of around 600. I currently own a 450D with the 18-55mm kit lens and I have a 70-200mm f4L lens. I want a lens for landscape and cityscape photography and am not getting enough use of the 70-200 (may sell). I have been recommended the following:

17-40mm f4L
24-105mm f4L

I'm stumped which one or others would be best and would appreciate some help, also if sigma, tamron offer any good ones?

Thanks very much