I'm afraid I have no experience of the Tamron, I guess very few people do at this stage. Reviews look promising though.
I do have a bit of experience of both the Nikon 24-70 and 28-70 2.8's though. The 24-70 is a great lens but if I were buying again, I'd save some money and go for the 28-70. Optically I can see no difference though I think I'm right in saying the 24-70 has nano crystal coating which I understand helps with flare. I can't say I noticed that to be a particular issue with the 28-70 anyway though. I prefer the build of the 28-70 and though you lose 4mm at the wider end to the 24-70, I never felt that the 24-70 was at its best there anyway so no big deal in my opinion. The biggest thing in the 28-70's favour is of course the cost saving, should be about £300 cheaper than a 24-70. Bear in mind though that since the 24-70 replaced the 28-70, all 28-70's are getting on a bit now and though they're built well, things can go wrong. My first one had a loose zoom ring and I've heard tales of focus motors dying as well. Maybe worth buying from a used retailer who offers a warranty just in case.