Assuming both are available and priced the same, I would pick the RX 6800. I used a 3070 Ti before, and I feel the GDDR6X is the most meaningless upgrade, and worst of all, the increased in power consumption over the non Ti version of the 3070 is unjustified. Nvidia chose not to release a 16GB version, but instead slap some hot VRAMs that I feel is a mismatch for a card at this level. Ray tracing at this level is nice, but the performance hit may be too significant for fast pace games, even with the help of DLSS.
In reality, the RX 6800 is non-existent. The only RX 6800 you can get is a screenshot or photo of it. The RTX 3070 Ti may be available, but woefully overpriced when the RTX 3080 is superior in every sense and not that far away in pricing.