This is a question we face everyday. Geely or Toyota? House-brand or Premium brand? It's the same thing.
Sigma and Tamron typically produce lenses that are cheaper than their Nikon counterparts. Sometimes, they are inferior in certain aspects. On other times, it may be superior. You gotta weigh the pros and cons and figure it out.
Read lens reviews, borrow/rent the lenses to try. It's the only way to know.