Someone already mentioned it... I'll repeat because it's probably the best lens for the money:
Get a 28-135... You can probably pick up a lightly used one for $300 or less, from someone who got it in kit . It's a nice walk-around lens equal to your 24-105 in image quality (maybe even a little better in some respects, less vignetting at the wide end), has fast/accurate USM, has IS and is mid-grade build quality. And it's an EF lens, so can be used on either of your cameras. Just doesn't have the sealing and more durable build of an L, and has variable f3.5 to f5.6 aperture, but is a whole lot better built than many of the more entry level lenses, some of which sell for more. Almost, but not quite as close focusing as 24-70L and 24-105L. Normally sells for around $450 new, but is very widely available on the used market.
I had one some years ago, replaced it with a 24-70L (for f2.8 and dust sealing, mostly), then ended up buying another 28-135 as a backup/walk-around lens (I paid $250 for a "like new" copy, including shipping, hood, and a 72mm B+W MRC 010 filter... but have to admit that was a particularly good deal). Hadn't used it much until recently, just for fun I put it on one of my cameras and used it throughout an event where lighting was good and dust wasn't too big a concern. I really can't say that the images look any different than 24-70L when it's used at middle apertures.