That’s a close call between those two.
the Sigma is lighter (a little bit, probably not that significant) and cheaper ($300 cheaper right now so about 30% - not insignificant). its long end is also closer to a true 600mm
the Tamron has closer minimal focusing, slightly better build quality, and its image quality at infinity is often superior (but sample variation comes in play here). I also founds it’s AF slightly more snappy on my Nikon D500 but no idea if it translates to your camera. But its long end is not quite 600mm (more like 560 if I recall).
So which one?
is budget a limitation? If yes, go sigma with no regrets.
is weight a big deal? If yes, go sigma with no regrets.
if you don’t care as much about the first two, is image quality at or close to infinity important (lots of birds in flight in your future)? If so, take the Tamron with no regrets. AF is also a bit better for BIF.
i ended up picking the Tamron in the end because I found it better built and optically superior (a very tiny bit) With snappier AF. But honestly, it was really a close call.