There is a *lot* of personal preference in this. We've got two really fast riders who'll be next to each other... one of 'em spinning away, the other one grinding away.
I've read (and seen) that people tend to pedal at a lower cadence than they should, in too high a gear, and get sore knees. That translated for me to "when in doubt, gear down, and listen to hyour knees." I'm a masher - I can't do 90 rpm without paying 'way too attention to my legs and saying "Faster! faster!" and I'm afraid I'll hit a tree. 75-80 rpm works, feels fine, and nothing hurts.
I'll do faster RPMs on the trainer but I have to think about it.