I bought the Tamron one a few months back, only used it a couple of times but pleased with it so far. It’s quiet and quick to focus and the stabilisation works very well. Image quality seems to be very good but I’ve nothing to compare it to in terms of a telephoto. Caveat to all this is I’m not a wildlife photographer so I’ve no idea how it would work for birds.
Good luck finding one though! The only place in the UK that had them in September / October was Ffordes. Even the grey dealers didn’t have them and there were no used ones anywhere at the time.