I won't tell you which lens to buy....but something to consider....
What lenses do you plan to own in the future?
I prefer to buy once, not buy item X now, then in 6 months or 2 years or whatever sell item X & then buy Item Y which is what I should have gotten in the first place.
If the 24-105 is your end lens, then buy it. If your longer term plans involve buying say a 70-200 then you might be better with the 24-70. Another thing to consider is whether you are going with the f2.8 or f4 all around or a mix. Not a big factor but something to consider. Also worth noting, where do the majority of your photos fall? if you have a lot falling in the 50-120 range you might find the 24-105 a better option than the 24-70 as you wont need to change lenses as much, if they fall into two distinct groups say 30-60 & 130-200 then the 24-70/70-200 combo might work best for you.
I just went through the whole which L lens should I buy first routine. I had the 24-70, 24-105 & both 70-200 IS lenses as my selection. My existing lenses are the 28-135 (40D kit lens) & the 50 1.4. A 70-200 is really good as it give me more reach, but really, I spend more time below 135 so do I buy something to do what I do now better or buy something new to expand my options?
To help, I split them into two groups, the 24-70/24-105 & the 70-200s, as really i was buying one of two types of lenses not one of 4 lenses. I did the pros/cons of the lens itself, vs the other lens in the pair and vs what I already have.
Ultimately I decided on the 24-70. Although the 28-135 may not be an amazing lens, it has more reach than the 24-105 and speed wise it is a bit of a mixed bag, f4 vs f3.5-5.6 though quality is a different factor, and I couldn't just straight sell the 28-135 either way, well not until a 70-200 comes home with me
. Even if I did get a 70-200 the 28-135 was staying put a while longer.
Now if only Canon would give us a 24-105 f2.8
Long story short... as much as you should focus on what you are buying now, do not ignore what you intend to buy down the road as you may find yourself doing more than planned.