1. Big gear "mashers" generally use a higher gear and lower rpm.
2. High cadence "spinners" generally use a lower gear with a higher rpm.
Larger, more muscular cyclists tend to push higher gears, while smaller body types lean towards spinning.
Popular wisdom these days is that spinning is better as it puts more stress on your aerobic system than on leg strength. Also, spinning reduces the chance of knee injury as their is less stress on the joint.
On my road bike I like to keep a 95-105 rpm cadence. This translates to the MTB on climbs and flats, and I believe I'm a better climber for it.