From the link that LTC posted, it looks like anything 1060 and above has identical performance for all but a few handful of formats. Not sure if that's software limitations or driver side issues because the 1060 does actually outperform it's bigger brothers in some tests.
The 1070 looks like a good pick though, it can generate the previews pretty quickly. Keep in mind that there can be other bottlenecks in the system that can keep the real time work flow from being smooth. If you can use a PCIe NVMe SSD in your system its a very good idea to grab one that is at least 1TB as a scratch/project drive for your edits.
I've built a couple of machines for editing for customers and normally core count, RAM, and SSD throughput were the bigger bottlenecks in the systems. For the RED 8k rig we put together we did a RED Rocket with a Quadro M4000 with 8-core 16 thread CPU and 64GB DDR4. That system was able to do real time 1/2 res (4k) at a little over 24 fps depending on what was being processed on the footage.
Ultimately it depends on what kind of footage you're going to be dealing with, and what kind of editing you want to do real time.
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