On paper, the 3070 is a good bit less powerful than the 2080 Ti:
The CUDA cores may look like they're greater on the 3070, but the 30 series includes INT32 cores with the number now since they can serve the same purpose as FP32 cores now. It's 50% INT32 cores, and 50% FP32 cores, so 2944 of each for the 3070. 2080 Ti doesn't include the INT32 cores in the count, so if it were to be listed the same way the 3070 is it's 8704. Every other spec is lower, other than the 3070 being PCI-E gen 4 and the TDP being 40W lower.
(Tensor cores were improved with the new gen which is why count is so much lower, possibly RT cores as well but unsure)
A used 2080 Ti will probably cost $200-$250 more than a new 3070 however, and for gaming they should perform similarly for the most part. You do need to consider the fact that the 3070 will sell out instantly to bots and may not be available to buy for a few months, and also may have the same capacitor problem the 3080 and 3090 have right now where games crash due to lower quality caps.