I have Dell 24" - full hd and LG 25" - full hd 3D. There is no point buying 30" on same 1920x1080 like my 25".
Size depends on your space and distance for viewing. If you have space go with 3x27" for surround vision.
1) Response time counts as well but technicaly under 4ms is impossible (you need to be as fast as possible)!
They advertise gray-2-gray... which is half time from black-2-white: so 2ms gtg = 4ms btw.
2) Contrast counts... they give dinamic one... just to have higher numbers.
3) Then there are I/O ports: DVI and HDMI (and if you have - DisplayPort) is a must!
4) Other improvements - like 3D, usb ports for easy access, tilt/rotate panel, etc..
6) Manufacturer: Eizo, iiyama, dell, benq, samsung, lg, asus. Aoc, Vizio (named randomly not in specific order).
7) Panel type: TN / IPS