From watching Digital Foundry's comparison video earlier, there seems to be practically nothing in it. The 1660 Ti is a few percent faster in some games, the 1070 in others. It seems to be mostly newer titles with newer engines where the 1660 Ti pulls away though - it creams the 1070 in Wolfenstein 2 for instance (27% faster), and pulls even further ahead if you enable the new 'adaptive shading' option.