the 15-85 on the 550 is meant to match the 24-105 on a full frame in perspective. I have did the move from the 15-85 to the 17-55 to gain the 1 f-stop in speed. Not sure it was that good of a move. It helps indoors, but outdoors, pretty much a lost differential - and certainly for landscapes.

My bet is that you would find the 24-105 not wide enough. test this by setting your 15-55 at 24 and walk around a bit, do you find yourself backing up? took at the exif data on your shots (if you use DPP it is in the "info" menu item on the photo). See you many of your shots are less than 24, just above 24? I be you find you shoot at the extremes of your zooms 80+% of the time and in the middle almost none.

As for the 40 2.8 vs. 50 1.8 - hands down 40 - nifty lens, nice and compact and very very sharp.

My suggestion is the 15-85. Very good image quality, good range, and for outdoors very handy range. If you find you want alot more reach, it time to consider the 70s to 200-300 options from canon of which there are several.