The key reason to get the new 24-70 L II is the resolution. If you own or intend to buy a very high resolution body, like the 7D (or any of Canon's 18mp APS-C sensors) or, if Canon releases it, a new 45mp FF body, you'll NEED the resolution such a lens has to offer to fully realize the potential of a high resolution sensor at anything faster than about f/5.6 (assuming aberrations are well controlled.) If you don't have a high resolution body, and don't intend to get one, buying any of Canon's newer Mark II lenses, unless they contain additional features like IS, probably won't do you any good...as lower resolution bodies won't really be able to take advantage of the better lens resolution.
The incredible increases in resolution is one of the reasons I've really been impressed with Canon's Mark II lens offerings (or new entrants, like the 8-15mm L fisheye, 70-300mm L). Owning a 7D myself, its clear that many older lenses simply don't resolve enough detail to take full advantage of the sensor resolution, even when there is zero camera shake. They certainly do tend to be expensive (particularly their telephotos...I'd love to get my hands on a 200-400 L, but its just a tad out of my price range right now), but the expense is worth it if you need tack sharp photos from high resolution cameras.