Pcpartpicker USA:
All monitors-> http://pcpartpicker.com/parts/monitor/
Then FILTER by:
- response time
- IPS, TN
- resolution
etc
*Again, investigate WARRANTIES (3-year recommended but other factors like dead pixels, shipping etc).
1) 25", IPS, 2560x1440, 60Hz (the one I bought my sister)->
http://pcpartpicker.com/part/dell-monitor-u2515h
2) similar to above (4ms reponse though) but much worse warranty (may kick yourself if it fails at 12 months plus a day)->
http://pcpartpicker.com/part/acer-monitor-umkg7aa002
You save maybe $90 which is a lot (approx 25%). Not worth the peace of mind to me.
3) ASUS-> (27", 5ms, 2560x1440, 60Hz)->
http://pcpartpicker.com/part/asus-monitor-pb278q
I haven't investigated the Warranty. If good, then it's slightly larger than 25" (2" makes more of a difference than you might think), 5ms response time isn't bad.
... above that it's GSYNC territory such as...
4)
http://pcpartpicker.com/part/asus-monitor-pg279q
$800USD is a lot of money.
GSYNC is awesome. Truly. However, I'm on a 27", IPS, 1440p and personally can't justify that cost. I've used it and it's definitely a lot smoother in many games (even the desktop).
*I've not investigated the quality or warranty either.
5) Might as well include THIS one:
http://pcpartpicker.com/part/asus-monitor-rogswiftpg348q
Since it's only $1350 you might as well get three of them. (seriously though, I'm debating getting a similar one when pricing drops. I'm debating the pros and cons of the ULTRAWIDE resolution though.)