The cost would be about extra $130. Does't seem a lot, but will rather use that towards other hardware unless is really worth it.
I'm in the process of building a new system that can run STEP Extended (all high quality options in the guide) with the Explorer's, Survival Pack and some mods from Weather and Lighting. I'm hoping to run Skyrim on Ultra.
So far this is what I have in mind: i7-4790K 4.0 GHz GIGABYTE GA-Z97N-WIFI Mini-ITX EVGA Superclocked ACX 2.0 NVIDIA GeForce GTX 970 4GB GDDR5 256GB SANDISK SSD Read 560MB/s. Write 290MB/s 16GB (8GBx2) DDR3/1866MHz Dual Channel Memory (G.SKILL Ripjaws X)
The two monitors in question: Acer H257HU 25-Inch LED 2560 x 1440 ($375) vs ASUS MX259H 25-Inch LED 1920 x 1080 ($224)
After the info given it seems that 1920 x 1080 would be my choice.