I have a 30mm f/1.4 and a 50-150 f/2.8. That is a great combo and hopefully I'll be adding a 10-20 f/3.5 in a few months to give me an outstanding set of EX's for my crop camera.
I think it depends on your other lenses as much as your intended use. The 50-150 is much more usable in doors and when dealing with people. When I put a 70-xxx lens on my crop body indoors, it never gets used, 70 is almost always to long. The 50-150 is perfect in these situations. The other nice thing about the 50-150 is that you have shorter focal lengths so you can get away with a little less shutter speed also. I did a side by side comparison a while ago of my 50-150 at 50mm f/2.8 and my canon 50mm 1.8 II at 2.8, and the sharpness was equal, colours equal, contrast equal, but much nicer bokeh on the 50-150. The downside to the 50-150 is that outdoors at places like zoo's it can seem a little short, so I've added a 1.4x TC to my bag to make it a 70-210 f/4 when I need more reach.
I probably will be getting a 70-200 f/2.8 also as I have a film setup and want something faster than the 70-300 f/4-5.6 i'm currently using. The 50-150 is the crop equivalent of a 70-200 on full frame.
If you don't have a 18-50 length lens the gap from 30mm to 70mm is too big. If you use your 30mm a lot then the jump from 30-50 is much more acceptable. The 70-200 works on full frame, but its a very different beast on full frame. If you compare the 2 lenses directly I think the newer design 50-150 comes out top for corner to corner sharpness across the focal lengths also. Sigma did 4 versions of the 70-200 so you'll find many opinions about "the 70-200" that might not be relevant to the version you are looking at. Sigma did 2 version of the 50-150 and as far as anyone can tell they are the same.
Both are good choices, personally I have gone with a 50-150 to match up with the 30mm f/1.4, if i went with a 70-200, i'd also be packing a 50mm prime with the 30mm prime.