980Sli or 980ti for 1440p

Hi guys I have decided I am making the leap to a 1440p monitor.
Namely this one:
http://ift.tt/1Lv3kJI

I will be getting a G-Sync later on down the line as I normally run a Twin monitor set up so its time to changing my monitors.

Currently I have a single 980 I got with my A51-R2, I am wondering should I get another one and put them into SLI or should I just get one 980Ti and...

980Sli or 980ti for 1440p

980Sli or 980ti for 1440p