If I was deciding between the sigma and a rokinon set I would buy the sigma and a rokinon 24, play with them both for a few days and then send back the one I liked least. And if I sent the sigma back I'd replace it with the other 2 rokinons.
When theres return policy's like Amazon have in the world, you might as well use it. All you have to do is print the return label and away you go. At least then you don't need to worry if you made the right decision or not.
I'm sure theres lot's of folk who could argue for one over the other but I think you need to decide what works for YOU best.
Thats my official answer. Unofficially, I replaced my rokinons with the sigma.