Weegee Bros :
/title pretty much
Where i live both the 144hz monitor and the 1070 cost pretty much the same.
1) Remember that not all games can take advantage of SLI, or do all that well with it. (for example, VR will not work with SLI or Crossfire for that matter.)
2) Your current GPU can do way better than 1080p 60Hz... It can even do better than 1080p... it can do 1440p very well.
Tom's states it as follows: Nvidia GeForce GTX 1070 -
Best @ 1440p | Best @ SLI | Good @ VR. Until AMD gets VEGA released, the only cards that can beat it, are even more expensive: GTX-1080, GTX-1080ti, Titan X (pascal), and even then 1440p can become child's play to them.
IF you go with a second 1070, you'll want to save up and get you a good 1440p monitor, maybe even with G-Sync.
IF you go with the 1080p 144Hz monitor, your 1070 can stretch its legs more.
Ultimately the choice is yours, no matter what we say. Personally, I'd look for a 1440p monitor around 144Hz... and the only one I see that doesn't get into 1080 - 1080ti pricing is: the
Dell S2417DG for 419.99 at NewEgg. It isn't IPS, but it is 165Hz, 1ms, with G-Sync... and a little on the small side at 23.8".
Of course, this assumes you live in the States.
http://pcpartpicker.com/products/monitor/#r=256001440&T=1,5&H=144,240&A=1&sort=a8&page=1