Used GTX 1080Ti vs new RTX 2070

Hello, I am looking to build a workstation PC for 4k video editing as well as advanced color grading and After Effects. I was initially aiming for a new RTX 2070 since it was within the budget but I found out a good deal on an GTX 1080 Ti Founders Edition, 200$ cheaper than 2070. The performance difference is quite similar(in Premiere Pro & DaVinci Resolve), so which one would you go with?

My main concern is how long would it last in a video & color grading rig, with moderate use, no overcklocking, stock.. etc. From tests I've seen, PP and Resolve are not using the GPU as much as I thought, even when working with 4k footage. It's only when encoding and using a lot of effects stacked with intense grading there is noticeable performance gains. I won't have a lot of projects requiring such an aggressive use of effects, but when I do I want a smooth performance and also future proofing my rig.

So, would it good idea to go for the used 1080 Ti? Or better get the new RTX 2070?

System details:

CPU: Intel - Core i9-9900K 3.6 GHz 8-Core Processor

Motherboard: Gigabyte - Z390 Aorus Master ATX LGA1151

Memory: Corsair - Vengeance LPX 32 GB (2 x 16 GB) DDR4-3200

Drives: Samsung - 970 Evo Plus 500 GB M.2-2280

            Samsung - 970 Evo Plus 1 TB M.2-2280

            Additional SSD/HDD

Power: SeaSonic - FOCUS Plus Gold 750 W 80+ Gold

What's everyone's take on the GPU? Since am aiming for a great 4k PC, and with the 9900K and spliting loads on dedicated SSD/HDD(as per recommendation) will the GPU make such a big difference in performance when doing complicated grades on 4k footage? I will be using both Resolve and PP.