Depends on how long you want to mine with the cards.  If you are playing the long game until the cards die (all with powered risers), then 1050 ti.  If you are looking at 3 years worth of mining then trying to sell 1070s. If you are looking at getting your ROI back then selling them to upgrade cards then 1060 (I am bias to 6gb because of better perceived resale value.) 

I have 8 750 ti cards running on two rigs been running since April 2014 only one bad card with warranty exchange during that time.  The 1-EVGA 1070 SC and 1-1070 SC2 have had warranty replacements already.