Well, if you're in 1080p 60Hz I honestly wouldn't do either and would go for either a 1060 or RX 480. A 1070 would be underutilized in that setup. You could take the money saved and apply it towards a 1180 or whatever is available by the time you do upgrade to 4k.
If you have a 1080p 144Hz monitor the 1070 would be a viable choice, if not slight overkill for some games.
As far as the possibility for future 4k, really the 1080 is the only option there, but expect to settle for around 45 FPS or drop a few settings.