I agree with the general consensus here. Don't buy a Mark I. If you insist on/absolutely must have a 28-70 f/2.8, the Tamron is a better choice. than a Mark I.
I just bought a Mark II that I'm hoping to take out this weekend, weather permitting, but I'm not ready to part with my 24-105, and I'm not sure I will be anytime soon.
On the other hand, you have an 85 f/1.8, a truly outstanding and underrated lens. That does fill the gap but you still may miss the high end of the zoom range on the 24-105.
One other consideration is which body you are using. If you are using a 5D3 or a 6D, there are image correction profiles available for Canon lenses only. These really help getting better results from less good lenses, but they do not, to my knowledge, exist for third party lenses such as the Tamron. In that case, I'd keep the 24-105 and save for the Mark II. It will be worth the wait.