There's been a quite a lot of this sort of discussion, and if you check similar threads here you'll see a range of opinions. I think it depends on your risk tolerance. If you have to go 1070 for some reason, personally I prefer the 1070ti since the extra CUDAs have a real impact for only slightly more price.
Overall just depends on how much capital you can afford tie up in equipment, your risk tolerance, belief in the future crypto, what you want to mine and whether it will be asic'ed or fpga'ed (some believe everything will) and whether makes sense for you to just wait for the 11 series cards dropping soon. Good luck.
Thank you for your reply. I have about $2K which I can invest in 4 GTX1070ti GPUs (I already have a spare PSU and MB). I have a lot of doubts about the future of crypto currencies, but I'm willing to give it a try
I'm just not sure if I should invest money in GTX 1070s or wait for 11XX...
There's no indication of how good the 11 series of cards will be, so it's anybody's guess- could be good, could be just okay, but just make sure you're okay with losing your money like Max said above.
As to how 'powerful' the rig will be, you're going to make approximately $166 according to Nicehash at current rates. Powerful is an extremely relative term and it's completely opinionated. What you should really be looking at is how much you might make and how long it might take you to break even at current rates. As of right now, it'll take about a year assuming things everything stays exactly the same, which most likely won't happen.
I'm personally staying away from GPU mining in general right now as FPGAs and ASICs are taking over algos left and right, but if you're okay with taking a bit of risk, considering the way things are going for you, go for it.