The 1070 runs quieter/cooler, uses less power and gets similar performance with the typical boost (~1.9-2.1 Ghz). However, it has a lower memory bus width (256- vs 384-bit) and fewer cores (1920 vs 2816). Roughly speaking the 1070 needs to be running ~800-900 MHz faster to match the 980Ti so if you overclock a 980Ti past 1400 MHz, then it's going to win by a small margin in most circumstances as 1070s don't tend to get above 2100MHz.
I doubt you'll notice much in the performance between the two cards.. so depends on how much value you put on noise, temperature and power usage (I don't think the memory is really a big factor). I'd not pay more than £250 for a second-hand 980Ti though. I just got one for £210, but had I been unable to get one under £250, I'd probably have paid the extra for a 1070.
Either way, you'll be happy
Thanks for this, maybe I am best off sourcing a cheap 2nd hand one in the wanted section. I need to check if I can run my graphics card in the bottom slot of my board. Otherwise my PSU is in the way.