Vegetto

it's best to buy the same ram, if not, both sticks will likely run at the specs of the slower ram. the 8600k does bottleneck a handful of multithreaded games past a certain frame rate around the 100fps  range regardless even if you oc, 6 cores/6thread just isn't enough for some games. But in your case, it's most likely just ram.

It's definitely because of single channel.
 
 
Pretty much all latest AAA titles can utilize 16 threads, especially games like BF1 and 5, which are known to be CPU heavy. I get 100% usage on a 9700k, so 100% on a 8600k is completely normal and expected. It's only 6c/6t.

RTX 2070 is good for raytracing and taking advantage of the tensor cores in CUDA programming and a bit better in terms of performance
 
While 1070 ti is best at the sheer price to peformance ratio

RTX 2070 will likely hold up better longer term than the 1070 Ti.  2 graphics cards one newer and one older that perform roughly the same now will start to have a gap in a year or 2 years as drivers for the newer model improve not to mention RTX 2070 will able able to use AI AA tech in the future that upscales resolution look at GN video to check it out. If you do 1080P you can save the cash but for 1440P I would lean RTX heavily.
 
If this purchase is going to make you sacrifice somewhere else in the build then the story changes but I'd say go RTX.