I've used both quite a bit with the NEX-7 over the last week and I think they are both very good. Tom is right that the main consideration probably is which angle of view are you looking for.
But I have found that although I normally have a slight preference for a 50 EFL over a 35, I am preferring the 24 to the 30 in this case. Although the 30 is sharp across the frame and the 24 can have some CA issues, I prefer the rendering of the 24 enough that it overcomes my normal bias for 50s. Although it is not the same look as using the ZM 35s on an M9, it is still very nice, and to me the color, the transitions from in focus to out of focus, and from light to dark, and the out-of-focus rendering, add up to a nicer look than the 30. I know you can correct color with profiles, and try to address the other issues, but I still prefer the 24.
But the 30 is very good too and pretty clearly worth the price. And the size and convenience are hard to argue with. I prefer slightly the rendering of the 35 Summarit on NEX-7 to this 30, but not enough to want to use the heavier Summarit and rely entirely on the legacy manual focus approach very often.
Probably the bottom line is if you are not bothered by paying for the Zeiss, then you should also pay for the Sigma. Either way, both are very good. The 50 seems pretty good too. The 19 seems fine.