Wide angle lenses are not, normally, very flattering for portraiture. They tend, for example, to make things like noses appear bigger than they really are! From the two you have suggested you will get more flattering portraits from the 24-35 although that's still too wide really. Having said that, you can get some interesting and quirky shots with an ultra wide angle (or even a fisheye) but they would be the exception. Depending on your sensor size and the space you have available to work in, a prime between 50mm and 85mm would be a good choice. Perhaps consider something like both a 50mm and the Sigma 10-20 zoom, you'd cover a lot of bases with that combo... Bet you'd use the prime for 99% of the time with your portraits though! Hope that helps a little.
Flashy