1070 ti is gddr5
1080/1080 ti is gddr5x ---> this generation or memory may bring future advantages
regarding density 1070 ti is not very far from 1080 ti, 1060 is ...
anyway density is efficiency too..lets say a 5x 1080 ti rig is ~3500 sols that is a 7x 1070 ti rig, the point is if your motherboards is 5x pcie, you need another board/system to put the 2x 1070 ti, additional system power consumption reduces overall efficiency.
assuming space is not a problem, but for home based miners with a room/basement allocated to mining (limited space) density is a must.
A gist of my understanding is just that it's lower power consumption for a higher bandwidth