Didn't centipede, missle command, and millipede all use a 2.5"?
I have a 2.5" Ultimarc trackball, and it works fantastic. My guests all enjoy it as well.
I don't really see why there is all of this tendency towards "arcade realistic 3" trackball. Centipede was in the arcade, and that was definitely in the top 2 most popular trackball games ever. Golden Tee being the other one. Centipede has a 2.5" and Golden Tee" has a 3" It's just about personal choice. A 2.5" works better for some games, and a 3" works better for others.
The ultimarc will cost you half as much. If you are going for cheap, it's a really great piece of equipment.