Originally, I went 1080 instead of 1080ti, cuz the price difference was like $200 USD, and the performance of the 1080ti was actually worse then the 1080 for gaming. Over time tho, that started to change, and the 1080ti eventually came out on top, but only by a little(in games). As far as rendering goes, and applications that require a lot of VRAM (basically everything other then games(with a few exceptions of course)) I felt my 1080's always came up short. I constantly ran into issues where 8gbs, was simply not enough for rendering some scenes. And going SLI made performance in most rendering engines awful. I regretted not dishing out the extra to get the 1080ti.
If you primarily use your video card for VaM, then I'd go with the RTX 3070ti. Cuz one day you may regret not getting it. And Considering the price difference is only $100 USD, If I was in your position, I would jump on the 3070ti when it came out. HOWEVER! if AMD's cards appear to be getting praise, and there aren't any driver complaints by that time, you may want to heavily consider an AMD as an alternative, as you will get a better bang for the buck.