Thia is going to come down to Ergonomics really.
I assume you can fit either on your desk top easily enough. So the next thing i would look at is Viewing distance. You would want to be sitting further back with the 24" set, i have had a quick google and most calculators agree that you would be best off being 3ft away from a 24" screen. While a 21.5 would want 2.5 ft. This is for a number of reasons but mainly eye strain.
im actually using a BenQ TFT 21.5" G2222HDL as we speak and there is no way on this earth that there is a huge differance in quality between this and the LG. The BenQ has won every review/group test i have looked up or read. Its 1080p but does lack HDMI outputs if thats a concern for you. Smaller screen size and still 1080p means smaller pixels so it will look sharper as well.
If you have any specific questione regarding the monitor please just ask.