Tamron 24-70 vs. Nikon 24-70...Worth the price difference?

Yes, sure.... !  :-))))

http://www.lensrentals.com/blog/2012/09/tamron-24-70-f2-8-vc-issue

Tamron is better value for the money - no question about it. Nikon is better built, sturdier lens with slightly better optical quality, more accurate AF and less sample variations. However, for being slightly better it asks a lot of extra money (like everything - Porsche isn´t 3x better than Audi S as the simple price difference may suggest, too.).

You are the one to judge if it is worth it for you. For instance, for me Nikon 24-70 was worth it. But not Nikon 35mm/f1.4 - I took Samyang instead (optically slightly better for 1/4 of the price! OK, no AF but I fogave it to Samyang for the price and optical qualities).