The 24-70/4 is a newer lens that I haven't used, but by all reports is quite sharp and capable. In comparison, the 24-105 is an older lens that's got some issues. Optically there are cheaper lenses that rival it. I am sure you can find some reviews online that compare them. The-digital-picture.com has their usual test and sample images done with each of them, too... where you can see for yourself how they compare. To me, the 24-70 looks quite superior at 24mm, the 24-105 appears to have a very slight edge at 35mm and 50mm, and the 24-70 has the edge again at 70mm (I only looked at the different focal lengths at f4, since that's probably the most demanding and least sharp... but you can compare stopped down images, too, if you wish).
If it were me, I'd go with the 24-70 and forget about a different lens for astrophotography for now. You don't necessarily need a fast (f1.4) lens for astrophotography.The f4 lens should be fine, I would think, and a lens with edge-to-edge sharpness is probably more important (big aperture lenses often aren't as sharp). It will be a long exposure, regardless. Instead of an additional lens, at least for now I'd recommend you get a quality tripod (which can be a once-in-a-lifetime purchase, if done right) and a remote release of some sort. Then start saving towards a wider lens. You'll likely want that for astrophotography and landscapes. I'd consider the 16-35/4L IS, which is also a relatively new and well regarded lens.
If those zooms aren't as sharp as you'd like for astrophotoraphy, then look at prime lenses. The EF 24/2.8 IS USM and 35/2 IS USM are both supposed to be quite sharp edge-to-edge. These are also newer models. As primes, they are a lot smaller and lighter, as well as less expensive.
Astrophotography doesn't demand autofocus, but you likely will want it for everything else. All the above will make for good landscape lenses, too.
For portraits, you might want some longer telephotos and larger aperture lenses. The large aperture allows you to blur doewn the background a lot, to make the subject really stand out or to minimize busy backgrounds when you can't control it, such as for candid portraits. Reasonably priced lenses you might consider are the EF 85/1.8 USM and 135/2L... Also the 100/2 would be very good for portraits. Among zooms, the 70-200/2.8 IS USM II is superb, but if it's too pricey Canon has three other 70-200s that are well worth consideration.
If by "close ups" you mean macro photography, the two Canon 100mm Macro lenses are excellent and full-featured. Tamron offers two 90mm, Tokina a 100/2.8 and Sigma a 105/2.8, all worth looking at, too. One of these might double as a portrait lens, too. Personally I think macro lenses tend to be a bit too sharp and unforgiving for portraiture, but a lot of people use them for dual purpose. All these macro lenses will get you to 1:1 magnification, which means you can photograph an object 24x36mm (approx.) with a 5D Mk III. In other words, you can fill the viewfinder with a US quarter (which is about 25mm in diameter).
If you don't need that level of magnification, can get by with something less (i.e., most flowers or larger insects like butterflies don't need more than 1:2 or 1:3 mag, at most), you may be able to get by just using some macro extension rings with some of the telephoto portrait lenses mentioned above. They can even work pretty well with the 70-200mm zooms.
There are lots of possiblilties! Have fun shopping.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER
The Sharpening routine. It is second to none.
