27 inch monitor 1920x1080 vs 24 inch monitor 1920x1200

Points:
1) You should usually try to keep the same resolution for multiple monitors, but you haven't given details as to your reasons for having them.

2) 27" for 1920x1080 is a problem because you can more easily see the space between pixels.

3) there are always deals on computer monitors

4) people with Widescreen monitors rarely scale the Internet properly. In particular, I use FIREFOX and CTRL+ mouse scroll to adjust the screen size. It remembers this for the web page until you change it.

5) 1920x1080 (16x9 ratio) is the new standard to match HDTV's. 1920x1200 will phase out. On the other hand, web pages are currently optimized for the 4x3 ratio screens so higher is better than wider in that sense.

Summary:
I wouldn't buy another monitor without a better reason. I recommend putting your money away and waiting for a better monitor with these basic characteristics:

1) 27" to 30" @ 2560x1440 ($500 or less)
2) 2000:1 or higher True Contrast Ratio (TCR is currently 1000:1. Dynamic CR is a useless value. TCR accurately represents the amount of light LEAKING through the front LCD filter which is why blacks look grey and colors are never as accurate as the older CRT monitors.)
3) better Anti-Gloss screen (prevents reflected light. I saw a demo of a screen where one-half used a new technique which is currently not in production. Huge difference!)

*OLED screens solve most of these issues but a reasonably priced OLED monitor is likely at least three years away.