GPU buying decision

i need new GPUs and i am considering 2x Vega 56 or 2x GTX 1070ti - both ASUS Strix version.

1070ti - cooler and quieter but slower
Vega 56 - still not out to buy and will potentially run hotter (with concerns that 2 of those might throttle after rendering for some time).

What would you recommend?

well if youa re waiting for aftermarket Vega 56, they will sure be running cooler then reference designs.

As for rendering, look at Blenders official results:

Vega 56 has descent performance even agains GTX 1080Ti, so should be good agains GTX 1070ti.