Radeon 5000 series currently have a large lead on NVidia 2000 series when it comes to ETH mining effeciency/profitability.  Assuming you will be able to get a hold of 6 3080's in the next 3 weeks at $700 each, they would have to do close to 100Mhz on ETH to be equivalent to the 5700XT's.  I seriously doubt they will even approach that number.

Now granted, i totally get that Nvidia GPUs are better at mining other algos, but let's be honest... right now, all the money is in ETH mining. Maybe that ends next week (i sure hope not) but you can only go with what you have.  Plus, losing 3 weeks of mining at these rates would be insanely crazy, especially since I already have it 90% bought.

Personally, I am much more eager to see what the Radeon Big Navi cards are going to be capable of. That's why I will likely be holding off on building my next rig until after they are released.