This totally depends on the circumstances. For example, if you are going to use the lens a lot, metal mount would be preferable. Similarly when shooting moving targets in low light (every artificially lit indoor environment is low light!) Sigma is better.
However, don't ignore the weight aspect. If you are looking to have a wide angle lens all the time with you, and don't shoot in low light (or shoot only non-moving targets in low light), and don't use the lens so much that the plastic mount would break, pick the Nikon.
I'm not familiar with the autofocus speed and accuracy of 3rd party lenses in the Nikon world, but at least in the Canon world either the speed or accuracy of 3rd party lenses is likely to be non-optimal. Official Canon lenses do not have the problems.
I assume the Sigma does not have image stabilization. It can more than make up for the difference in lens speed, but only if shooting non-moving targets.
Also, this goes without saying: ensure both lenses are compatible with your camera. (I didn't; my answer assumes you already did your research.)