If money isnt an issue I would go with the Nikon mainly because it is Nikon.
Is it worth the extra 1000$ over the Tamron ?
I dont think so.
So first the Sigma, its a great lens that works actually pretty well with Sigma's TC.
Sigma's IQ, OS and AF speed are considered lower then Tamron and Nikon but that doesnt mean its a bad lens, its very sharp and very good but its almost same value as the Tamron new is so..................
I was looking for a 70-200mm lens few months ago, the Nikon version was not even a consideration because its 1000$ more expensive so the split fell between the Sigma and Tamron.
I chose the Tamron because its sharper, has a better resale value and overall considered roughly equal to Nikons lens (some say its actually sharper) and its VC is amazing!!! all for almost 1k cheaper.
Only negative is that I dont know how well it works with a TC, I am in the process of looking for a TC myself.
For me my Tamron is everything I was looking for and more, very sharp, fast AF and as I said very efective VC.
They say QC of Tamron has improved dramactically, well when I got the lens (new) it wasnt all that sharp so I had to fine tune it (-20) to get the results I wanted.
I went to Tamron service centre and 2 days later got the lens back, now its tack, tack sharp, just a jewel!
If I had to go back and consider what to get again I would choose the Tamron again.
Just to add I also bought the Nikon 50mm 1.8G new and also had soft pictures with it so I cant say my experience with the tuning of Tamron or Nikon should be taken too seriously but it is what happened.
Good luck and enjoy your trip
