Wow! The very first post!! Congratulations!
Keep the 24-105 f/4L. It's a professional grade lens. Not only is is sharper, having the fixed aperture is a nice thing to have. You can be at f/4 @ 105mm, 24mm 58mm, whatever. With the 18-135, as you zoom out, the lens automatically stops down to f/5.6. The only time you will get f/3.5 is if you are at 18mm. The problem that you will have to decide is zoom capabilities. You want to go wider, but if you are constantly zooming, that causes issues with the lenses you want to buy. For instance, I would HIGHLY recommend the older 17-55 f/2.8, which is the "24-70 f/2.8" for crop bodies. Right now your 24-105 is acting like a 38.4 - 168mm. (Focal Length x 1.6 = Angle of View Change / Crop Factor). The only downside to the 17-55, is that it's a crop body lens only.
So I get with what you are saying, this is a tough call. Keep in mind that if you ever decided to go full frame, the 24-105L is a full frame lens. You'll get your "wide" back if you upgrade. As for the 70D, I've seen people complaining about noise issues. So while it may have all the gadgets & gizmos, your results may not be that great. Perhaps a Canon user will chime in.
