In general, I do best when all the text I view during a session at the computer is rendered by a single system, like Aqua or Gnome/GTK. My visual system is defective (exotropia and "lazy eye") but not so much that I have any restrictions on my driver's license. I can read 12-point text on OS X (and probably 10-point text, too), but when a system can handle it without complicating my life, like Carbon Emacs 22 can, I prefer 18-point text, and part of the reason that I do is probably that there is more visual information per character. (I'm a little far-sighted, and my head is usually 4'-5' from the monitor.)