Thanks for the info. What is it about the Nikon that makes it win hands down for you?
I assumed you were shooting with the D810 since I didn't see a D500 in your signature. I'm not afraid to use a third party lens, especially Sigma these days. They've been making some excellent as of late. I played with both the Nikon and Tamron in a local store and the IQ seemed comparable, but the Nikon's VR felt much more confidence inspiring. Plus, the extra reach of the Tamron seemed minor at best. The Sigma contemporary version I handled produced the same FOV at 600mm as the Nikon did at 500mm, so there was no benefit there... I think I'm going to end up renting the Nikon and Sigma sport version and seeing which works better for me. CA doesn't really bother me as LR removes it very nicely. At least half my lenses produce CA under poor conditions, so I'm used to it...