a prime for low light would always be appreciated. i've seen the 24-70 myself and all praise on it. it's a wonderful piece of equipment. ultimately, if u can make do with ur zoom lens for portrait, why not?
sorry but i can't give comments on the tamron 28-75.
I would advise u to rent it out first and try. No doubt its a piece of good glass, but personally i couldnt convince myself to part w that kind of money. Of course if u can well afford it, go ahead and get it, its one of the best out there.
Why would you say that?
Between Nikkor lenses and the (up and coming) 3rd party brands, you shouldn't be left wanting.
From ultra wides to really fast primes to extreme telephoto, all also have ;p
only whether your budget can tahan or not... hehehehe