No reason to go with the 24 inch. It is cheap, and will die sooner, and big chance in 2013 you won't find a 23.5 inch 16:9 either. It's probably 24.3 inch by then, which means you have to recut the bezel anyway.
I would go for the much better viewing angles and built quality of backlight electronics. Tiny kids, tall basketballers, and 6 X-men side by side, all can see equally well on the 20 inch, whether it is vertical or horizontal mount.
I would also prefer it for desktop use. 1200 pixels vertical is much better for internet, word, etc than 1080 pixels.
Did I say the picture is bigger on the 20 inch? If you keep 4:3 aspect, it will look bigger, definitely!