For the argument of 1080 over 1070
The Price:
The 1080 costs a whopping
630 Pounds
The 1070 where a 1070 only costs
435 Pounds
That is 225 Pound Difference
So Price Wise the 1070 Wins
The Specs:
In the Specifications, the 1080 obviously has better spec with higher core and boost clocks as well as having a higher memory clock among other things
So the 1080 Wins in the Specs
The Performance And Benchmarks:
The GTX 1080 is around 25 percent more powerful than the GTX 1070 wich can make a big difference when it comes to games and any graphically intensive workloads but you do need to consider whether or not your other components will throttle its speed and or whether it is worth it because if you have a 1080 and an intel Pentium they are not exactly balanced and therefore the 1080 won't perform as well.
Pice to Performance Ratio:
As seen the GTX 10 80 clearly stands out as the card with the most power and performance but what needs to be considered as well is Do you need that extra performance? because if you do not needing that extra performance then you can save a couple hundred pounds to get a card that is much cheaper but still has great performance.
In my opinion the 1070 has a better price to performance ratio than the 1080 because the 1080 is not standing out like a sore thumb racing far ahead then the 1080.
The Conclusion:
In conclusion, the 1070 is a better buy as it is cheaper as great performance and has a better price to performance ratio than the 1080. If you realy need the extra power and you have tge extra bucks lying around then go for the 1080 but In my opinion the 1070 will do just good and you save yourself some money you can use to but better things in life and games.