Are you using a full frame camera body?
I believe the "DG" nomenclature of the Sigma indicates that the lens only covers the APS-C sized image circle. Not sure though...I've lost track of all the abbreviations over the last few years' time. I think for a walkabout type of scenario, the focal length flexibility of the zoom would be the thing I would like the most about the zoom lens. I own a 24/2.8 AF-D...I don't use it that often any more, since I have its focal length and its aperture in several good zoom lenses. The 50mm prime really has the low-light advantage over the much slower f/2.8 zoom lenses, when a fast shutter speed is needed, and when the added light of the 1.8 aperture makes autofocusing more-accurate and more-sure under really demanding situations.
It seems lately EchoingWhisper that you're very concerned with the selection of your upcoming lenses; I recently read a web essay about the way many amateur photographers spend a HUUUUUUGE amount of time, and effort, and research, in selecting their equipment, and as a result, they become VERY defensive about their kit. As the author put it, you can criticize the photographs these people make, and they will reply, "Ehhh, just a test shot I did," or "Just a snapshot of my daughter, no big deal." HOWEVER, when one criticizes the equipment these people have assembled as part of their kit, they become extremely defensive, or even downright combative. Anyway, not to imply that you are like that, or have behaved like that, but I have seen and responded to a number of your posts comparing four different, specific lenses for Nikon use. The majority of lenses available these days are quite decent. MY suggestion is to look for an outlet wher you can buy used lenses, at a good savings. Possibly via mail or internet, in your own country, and thus save a lot of money, and allow yourself to buy and try lenses without paying full, new retail prices.