I guess it comes down to what you're shoping for.
If you're looking mostly for Disney stuff, well, obviously WDW is the best choice.
If you're looking for high end mechandise (and I mean high end as in BIG BUCKS) then Vegas is you place, they've got every top designer on the planet.
If you're looking for shopping to be only a part of your vacation, and you're also looking for other activities to do as well, then of course you need to look at that too. WDW is more for families or "wholesome" entertainment, Vegas is definatly for adults looking to party (for example, waking down the Strip, guys are paid to hand out flyers for hookers and strip joints, and the flyers include pictures!

). Both locations offer great restaurants, Vegas is arguably a bit better in this area, as many of the fine restaurants in the Orlando resort area are of course on WDW property.
If you're looking for pure shopping, nothing else, may I give you a third option? Chicago! We've got TONS off shoping dowtown, especially on Michigan Ave (including a
Disney Store). There are several indoor malls, if you plan on going in winter and don't like the idea of being outdoors. There is also a ton of non-shopping things to do, too.
Anyway, good luck. I love both Vegas and WDW, so at the end of the day, I don't think you can go wrong with either option.