I find a UO 6mm orthoscopic is the king of my planetary EP's, but it was an older university optics model from 10 years ago. God, the sweet views that has given me.
Bought a 4mm UO late last year, new model. Fantastic in +5 seeing, crap for most of the atmospheric time though.
In short- if you are going to go as low as 6mm for planetary viewing, you might as well spend. Second hand can be fine too.
All depends on what you are plugging them into as well, plus your own eye conditions, and so on, and so on.
Good 6mm contrasty EP, so versatile- think planetary Nebulae too, and tiny globular star clusters so dim that they need a lot of light through well corrected lens elements kept to a minimum.
Most seasoned astro hogs will have a few ortho's. even the ones with panoramic TV's- just so they can get a feel if nothing else.
