Between Sigma and Tamron, I would go with the Tamron 70-200/2.8 at about $1,200.
The single reason (for me) is that the zoom ring turns in the same direction as Nikon lenses. With a Sigma zoom, I was constantly turning the zoom ring in the wrong direction, as I zoom with muscle memory. When I zoom, I don't think turn left or right, I think zoom wide (shorter FL) or zoom tight (longer FL), then my hand turns the zoom ring in the appropriate direction. Those of you who shoot sports or action will know what I mean.
But, with the Z cameras out, you may find deals on used Nikon F mount 70-200/2.8 lenses, as people move from F to Z cameras.
You can find great deals on AF lenses, BUT, they cannot be coupled to the Z cameras, if you ever get a Z camera.
Only the AF-S and AF-P lenses can be coupled to the Z camera, via the FTZ adapter.
If you can get away with the f/4 lens, the Tamron 70-210/4 is both lighter (about half the weight of the f/2.8 lens) and cheaper at $600 (about half the price of the f/2.8 lens).
If you are shooting for a long time, HOURS, the lighter f/4 lens makes a big difference in how tired and sore you arm gets.
I personally use the Nikon 70-200/4, and got it primarily for the lighter weight. Shooting two sequential field games (football/soccer/lacrosse) = about 5 hours of shooting. And unlike some of you guys, I am not a youngster, weight makes a difference.
Note, the Tamron 70-210/4 has a FRONT positioned zoom ring. At first I had my doubts about putting the zoom ring in front. But in actual use, it worked very well. The lens is in my left hand, and my fingers are on the zoom ring. The zoom ring turns very easily with my fingers. THAT is the key. The front positioned zoom ring, has to be easily to turn with the fingers. If you have to muscle the zoom ring to turn it, you can't do it with your fingers, and the zoom ring would have to be further back, where you can grip it and use your hand/arm to turn it.