There is nothing wrong with either but if you do 1070 the 1070 ti comes out soon. But as Richard said AMD improves over time with drivers and feed back. Hell Ryzen is great for gaming now after lots of driver updates and game updates.
Hmm not sure though pricing of them seems too be higher than normal
As of now GTX 1070 £399
GTX 1080 £500
GTX 1080TI £600 and up
I'd rather pick a 1080 reason it's been out for a while issues are sorted