I base it on how much I'll be using the secondary monitor.
If you are using it as a sometimes secondary monitor, maybe throw up a stream on it once in a while or a a few documents you'll look a when working: Basically any ole monitor will do. I would try and find one about the same height (psychically) though. It makes it more comfortable when moving the mouse around.
If it's a frequently used monitor on a work system or something you are doing a ton of productivity type work on, using dual monitors >50% of the time, I'd prefer them be the same.
In some cases a different monitor is even preferable though. For example I have an old 4:3 monitor I keep around and if I'm doing a lot of word document stuff I'll hook it up as a second, since 4:3's are generally give you more vertical space.
It looks like the one you have now is silver, so if you wanted them to match color wise jsut for looks purposes that will be a little harder. Though you could maybe get something like this one which has a very thin bezel so there's not much to see anyway:
AOC I2267FW 22-Inch Class IPS Frameless/Slim LED Monitor, Full HD,250 cd/m2 Brightness,5ms,50M:1 DCR,VGA/DVI - Newegg.ca
Or this one a little more money but is another 23"
AOC I2379VHE 23" 50,000,000:1 (DCR) 16:9 LCD Monitor, 250cd/m2, Frameless IPS with HDMI - Newegg.ca
Also don't forget to check what inputs/outputs you have available.