another reason is temperature, GTX970 works at a very high temperature (75C with 25C ambient, even with the three coolers, insane!) , I don't like that, as an electronic tech, i know temp kill rigs badly!, it's only a matter of time!.
Don't get me wrong Gigabyte GTX 970 G1 where fantastic cards, I love them, power density is awesome, they are extremely stable, beautiful (i know, I'm a nerd, can't help it) but when it comes down to power efficient, it's like 40% more than 750Ti (including the rig overhead!).
There's nowhere near 40% difference in efficiency between a 750 Ti and a 970 because they are both Maxwell cards. The only way you'd get such a huge difference is if you use a memory bandwidth/latency hard algo (old lyra2) but for most algos a 970 is about 2.7-3.1x times faster than a 750 Ti which reflects their power consumption.
I have GV-N970WF3OC 970 cards which are virtually the same as the G1 ones and they never ever go above 60°C with 8cm gap between them.
The 1 fan 970 minis on the other hand would go 75+ if I'd let them but I use a 70°C temp target on them.
I plan on measuring the hashrate, power consumption, temperature for all relevant algos, for all the different type of cards I have with different power targets for a long time, I think I'll get to it later this week and share the results.
also, mining, kills hard drives like crazy, unless use SSD which are expensive here.
Not unless you use terrible HDDs to begin with like WD Green series which keep parking the head after 8 seconds of being idle and are only rated at 300.000 of these parkings.
60GB SSDs are not expensive anymore (and preserve their price better) and I'm still below 1TB total writes on them after 1 year of constant use running several wallets.