I quick like the 24-70 focal range on my APS-C cropped D90. i also have the Tamron 17-50, but always find it too short for portraits when walking around.
The ultimate downside is the weight of the 24-70, its damn freaky heavy and have to be balanced with a battery grip..
it's about the same (some say better) as the 24-70L, but with added IS. the thing is, it has soft corners, though not noticeable on a 1.6x crop. you only want the f/2.8 if you want a stronger bokeh, or when your subject tends to move more (if it really moves a lot, then i seriously doubt the advantage of just having one stop more).
so i would say, go with the 24-105L if you like its weight and focal length.
To optimise the efficiency of its hood, at 70mm its retracted back to its shortest length and thus the hood provides a deeper shade for it.. Not exactly like a Nikon, their zoom ring rotation direction is different but this design is a reverse zoom..
You may find the 24-70 too restrictive/ not wide enough , as you know 24mm is 24 X 1.6 = 38.4mm. IMHO i would prefer either the tamron 17-50 or Canon 17-55 for a walkabout lens
With a crop factor, I think the camera offers a pretty good range...with the low aperture value its also pretty easy to use it handheld at night at 400 ISO, you can still get decent pictures with it. As a walkabout, good lens, but if your shooting landscape from quite a distance (ex from hilltops), then I suggest you get some lens with a wider telephoto range.
for me i will keep my tamron 17-50 than add another 24-70 to my collection of lens... norm i no not need 17mm wide end however when i need it, it doesnt justify me buying another 10-22 to cover that wider range i might occasionally need...
how i wished there is a 24-105 f/2.8 IS USM... best of both worlds for me...