If budget limited or having the very best was a factor i'd say wait a month. Brand new product lines for 2016 from Samsung, LG and View Sonic are coming to market now. Also we are seeing more much cheaper IPS 144Hz 1440p hit market like:
http://www.newegg.com/Product/Product.aspx?Item=9SIAB714894154&cm_re=144hz_1440-_-9SIAB714894154-_-Product using the same panels as the higher end/priced 2015 Acer and Asus models right now. By August we will see more off brands using the 34" widescreen and 32" standard IPS panels soon with 144Hz and DisplayPort 1.2a (at least).
Right now, the Predator X34 is stunning and the best gaming screen IMO assuming limitless budget and SLI 980/ti/1070 or 1080. I am biased toward ultra-wide curved, however, and while ideal for MMOs and many games, it is very subjective. In regard to refresh comparisons, even a fighter pilot couldn't differentiate Gsync at a steady 100 FPS vs 165 or even 1 million in a real world gaming scenario, not a lab study. The X34 like acer monitors in general have quality control issues and you need a vendor that will allow unlimited returns to assure humanly bearable light bleed- if unlucky.
Not that Gsync is even much of a factor past 80 FPS at the low end. And that is after that fact no SLI or tri 1080ti setup is making 80+ minimum FPS rates an issue for the X34 or worse- true 4&5K- for a year at least in a modern studio game. This also means future 4K gamers and even 1K/2K gamers (hopefully VR) can do fine/better with future budget cards with the correct monitor (free vs Gsync- thanks to Nvidia's protectionist greed).
Until DisplayPort 1.4 (or even 1.3) panels hit market, today's best reasonably priced monitors all have some major compromises until very soon, meaning hopefully by next month to have stock for Christmas. Having an IPS panel, unnoticeable response, 144Hz(120Hz 4K) and 10-12 bit HDR and even 5K (which may be a superior curved ultra-wide gaming resolution for some) is endgame for a few years in terms of major long pre-established noticeable upgrades and it is this summer's top end release depending on LG and Samsung's decisions.