Definitely spin slower on the 'bent over the MTB... and go much faster.

:eigh and Hash. You are right. For several years I tried to count my cadence while biking, and always thought I was 100 - 120. Then got a meter and found it was only 90 - 95 on average, and while looking at other bent riders' cadence I seem to be faster than most, because of my short cranks and leg weakness. . So I suspect that unless you have a meter/counter, we all tend to overestimate our cadence.
Not that it matters - I just ride at whatever speed or cadence is comfortable for ME at the time, faster uphill and a bit slower on the flats maybe.