Both are totally different lenses. To answer this, you would need to determine your photographic style. Personally I'd get both, or just stick to a 50mm f/1.8 for portraits.
In most situations, the lens i'd use would depend on the location I'm at, and how much space there is.
If you don't know how differently these 2 lenses behave, don't buy, rent first.
They are totally different lenses and how you use them is quite different. So how to compar Apples and Oranges? Its like comparing a commercial jet to a BMW, for which its quite obvious their difference. Surely you can buy a BMW to travel all the way to China but would you? it really depends on you.
Depending on the camera body you have and your personal shooting style, 24-70 can be a great general purpose lens. Wide enough at 24mm and with just enough reach for nice portraits at 70mm.
70-200 f2.8 is not something most people will mount on their camera for walkarounds though.
Am using 24-70 as my main lens, cos the range is quite good for most things. 70-200, i would use it more for sports, car races, aeroplane shows etc, generally where u need to zoom in a lot more. Have been wanting to get the 70-200 for awhile but at the end of the day i hardly shoot something with that range...