RTX 2060 or GTX 1070 Ti?

What you need to consider is that the RTX 2060 is on par or superior to the GTX 1070 Ti in all but amount of VRAM. Some pros and cons for you to consider:

RTX 2060 Pros:

• Newer architecture, possibly better long term driver support. At the moment performance with the 1070 Ti is tied except in some newer games such as Shadow of the Tomb Raider, where the 2060 pulls slightly ahead. If you recall the situation with the 780 Ti vs. 970, the former didn't hold up as well as the latter as time progressed. It remains to be seen if we will see a similar situation with the 2060, as drivers mature and devs learn to better optimize for the newer arch.
• RT and Tensor cores, support for things like DLSS and adaptive shading. Not much going on at the ray tracing department right now, but that may change down the line. (Quake 2 raytraced being a notable exception). It's probably questionable if the 2060 will be able to be up to the task with its limited RT cores, however. If you use your PC for other things than gaming like content creation (video editing, 3D rendering, etc.), some software packages have already announced support for the Tensor cores in the RTX cards.
• Better video encoding and live streaming quality and performance.
• USB C-type link interface for future VR headsets.

RTX 2060 Cons:

• Only 6GB VRAM vs. 8GB in the 1070 Ti. It's hard to say right now how much of this will turn out to be a problem in the long run. I'd say that if you only game at 1080p, you probably shouldn't worry about this too much. However, if you are thinking of upgrading to 1440p or 4K in the future, this may be a concern. On the other hand, if you do content creation, 6GB can be a serious limitation. I do video editing, and rendering native 4K video with effects/colour grading etc, uses up all the VRAM I have. In this case, I am glad I have 8 GB of it. Same for 3D rendering, etc.
• May end up being overpriced if the rumours of a non-RT "GTX 1660 (Ti)" being produced are true, and you don't care for fancy ray tracing.

I'd say it depends on the games you play. In TechSpot's 2060 review the 2060 beat the 1070 Ti by 4% on average at 1440p. The 1080p numbers, available on the Hardware Unboxed Patreon, aren't much different. The short of it is, in some games the 2060 is around 1080 performance, and in some it's closer to the 1070. It's above the 1070 Ti for most games.