Buying three 1070Tis = £1800, two 1080Tis = £1700.
Although, the price of 1070ti is a £100 difference but you can get 3 GPUs. so price 1070 ti cheaper
The three 1070Ti will get £210/month while the two 1080Ti will get £180/month.
income/month 1070ti greater than 1080ti
the 1070Ti will be easier to power with just a 1x 8pin connector (217W), while the 1080Ti will be more troublesome with a 8+6pin or 8+8pin (250W),
1070ti more energy efficient than 1080ti and I suggest buy 1070ti , great in mining Zcash, Neoscript.