i'll go for a 1070, if you sell your 3 750Ti, that should pay for about 50% of what a 1070 costs.
that's what i did, i sell all my 750ti's (i had like 30 ) and buy 1070's my power bill decreased like 30%
750ti where fantastic cards, the best ones i ever used, but their time for mining is over IMHO.
The number checks out.
On one hand there's the difference in fabrication process which says a lot:
GTX 750/750Ti/760/770/780/780Ti/Titan/Titan Z - 28nm
GTX 950/960/970/980/980 Ti/Titan X - 28 nm
GTX 1060/1070/1080/Titan X(P) - 16 nm
GTX 1050/1050 Ti - 14 nm
But, every card has their advantages and disadvantages (e.g. different efficiency) on a per algo basis.
Considering how old the 750 Ti's are, I'm still making ~70% of what I make with 1070's watt for watt.
But it's not as easy to sell them.