Photography is not your livelihood. If cannot afford now, hold and save. Dun force a compromise. Then each time, you use, you will still dream of your WANT lens and feel tak shiok. Wherereas, if dun buy, you will still dream but it will be a shiok feeling.
Tamron 24-70 is like Tenna/Camry as opposed to BMW5i. For most shooter, in practical situation, it will get the job done just as good. I dare say, if the lens design and construction remain exactly the same, but the Nikon make the "Tamron" and vice versa, the review will be much much positive. (you know, 3rd party lens cannot be so good one and cheap, there must be some faults and hence people start finding, or worst still, become a parrot amplifier without even touching it!.........)