It’s a question that occurs to every serious photographer with a dSLR getting his first pro-grade lens: should I get the 24-105 f/4 “L” IS USM or the much-costlier 24-70 f/2.8 “L” USM? And the answer depends on you - and what you plan on shooting.
Both are fine lenses - drastically better than any non-“L” lens. Both are dust-resistant and weather-resistant. Both are very-well-made in ways that non-“L” lenses aren’t. Both cover the range from wide-angle through some degree of telephoto - whether on a full-frame dSLR or an APS-C one.
I’ve had both - and now have the 24-70 f/2.8 - and here are my observations from extensive use of each. As a walking-around lens, the 24-105 is compact and lightweight - while faster than most similar zooms in the film era; it shoots great photos - in decent light, but really isn’t suited to no-flash photos indoors or to edgy camera settings to create special effects. But - for either special image effects depending on edgy camera settings or for indoor no-flash shooting - the 24-70 f/2.8 is almost essential.
That means that - if you plan to shoot weddings - get the 24-70 f/2.8, as many if not most churches prohibit flash inside. If you plan to shoot movie-poster-like photos, the 24-70 f/2.8 again is about essential - even in bright sunlit locations. Otherwise, you’ll enjoy the 24-105.
Worth noting: the 24-70 f/2.8’s first version shoots fine photos even with edgy camera settings - and made a lot of money for a lot of wedding photographers.

Mitt Romney’s son Tagg campaigns for Mitt, Burlington, N.C., Oct. 17, 2012.
Shot with 24-70 f/2.8 “L” USM.