Which is more exercise beach cruiser or a mountain bike?

Answer 1

For an equal amount of effort from the same rider, a mountainbike ought to be faster. It's likely to be lighter, offer a better riding position and multiple gears will allow the rider to ride more efficiently.

Answer 2

It's really much more down to how you ride it than which type of bike it is. Slowly pootling around on a MTB won't give any more exercise than slowly pootling around on a cruiser. If you're serious about getting some exercise I'd recommend a MTB. Odds are far better that you'll be able to find an efficient riding position on a MTB than on a cruiser. If you don't know how, go by a bicycle store and have them set the bike up for your size so that you get a decent riding position. Don't forget that eventually it'll be your commitment that'll make the difference. If you don't do the riding and push yourself a bit while doing it it doesn'y matter which bike you're using.