If you Already have a 780Ti, grab a second one for SLI quick before they are GONE (w/900 series intro 770, 780, and 780Ti were discontinued). Prices on 780Ti's have dropped to about $400 on NewEgg.
You are, of course, planning to game at 1440P right? If your still at 1080P I don't see much point in either GPU upgrade as a single 780Ti will already max out most games at 1080P.