I still remember the first time I sat in front of a 21" CRT running 1600x1200 resolution... It was a system I was professionally optimizing for someone who wasn't computer literate so I had to just pretend I was used to such a thing. ;D
I actually like the pixelization effect from viewing low-res graphics on displays that are way too large for them, but I know not everyone does. ;)
However, size should also be a factor of practicality. The 23" widescreen LCD on my current system weighs a heck of a lot less than the 17" CRT on my previous system, which is even heavier than expected due to it being a flat-crt where the flat surface is actually just a huge, thick, glass lens in front of the real CRT. (On the plus side, no static discharge if you touch the surface!) Where you intend to put it is also a factor. If my widescreen monitor was even just one inch bigger, it would be too big for the desk I've got it on. :P
I say go with the 18" and save the 21" for a rig that can take better advantage of its high-resolution potentials.
--- Kris Asick (Gemini)
--- Pixelmusement Website: www.pixelships.com
--- Ancient DOS Games Webshow: www.pixelships.com/adg