Well a 17mm on your nikon acts as a 25.5mm. If you get the 24-105 I mentioned, it'd look wider than 17 on your Nikons, and still have the 105 telephoto. I think you'd be disappointed by the 40mm limit on a full frame. It'd be the equivalent of a 27mm on your Nikon. I think the 24-105 F/4 L is a good option to start with on a Canon foundation.