I would think if I had to choose one of the following it would be the zoom. Because you want to be far enough removed from your subject to give them room to move but close enough to be in control. Ideally, something from 80mm to 135mm would be good for portraiture. The 30 would put you nearly in their face and give them the doggie-nose syndrome (highly technical term).
If you are doing studio portraits I would go for the zoom... if it's portraits like you taking photos of the people around you in everyday life, I would go for the Sigma... but for studio portraits there are better options than both.