Ergonomic studies with respect to computer monitors suggest that you should be viewing with the screen just slightly below the horizontal level of your eyes. I see no logical reason why it would be different with a TV unless the fact that you are farther away makes a difference. I can't comment on that. So I would second what Dtsag says and suggest the 21 " stand.
Suggestions on HDTV viewing height
I just bought the Mitsubishi 73" DLP and was looking for a A/V stand that would house my audio components and of course, the TV. Salamander makes a few good stands but the triple shelving (65.75" W) comes in two heights 31" and 21". I feel the 31" stand will place this large tv up to high slightly above eye level whereas the 21" is almost eye level. My viewing distance is 13.5' and the room is 18 by 15. Any suggestions would be of help