atomicWAR :
usually the single best card you can get is the best choice. However in titles that support SLI 2 gtx 1070s will out perform a single gtx 1080 in 2k/4k. Honestly if your going 2k I would just get the gtx 1080 if your going 4k though Then I might consider 2 gtx 1070s. I know this recommendation won't be popular on toms but as a 4k/SLI user myself (2 gtx 1080s) I can say for a fact one GTX 1080 is not enough in a lot of games for max settings with good filtering and AA. 2 cards is not always enough but it helps a lot. I play a lot of SLI compatible titles though I can't say if you do or not. Regardless if you don't mind the short comings of SLI then 2 gtx 1070s will give you a better experience at 4K then a single gtx 1080 when supported. I also find SLI usualy is only supported in games that DO NEED 2 cards to max out. So SLI/CF can well be worth it if your a max settings with high resolutions enthusiast like myself.
thanks for ur time , u mean a gtx 1070 sli for 2k is overkill so u consider to get a single 1080 ?
the problem is that i saw a lot of benchmark for gtx 1080 on 2k and some games the 1080 cant stay at 60fps it will go down to 50 or even 40 fps like the division ,gta v
iam not planing on upgrading my gpu for like 3 or 4 years so what u suggest ?