True spinning tends to let you go farther without getting tired. It also often provides a smoother more proper pedal stroke. Mashing tends to be jerky, with all the effort from the quads on the down stroke. Spinning is more circular and even. If you can afford it, buy a cadence monitor. Either one with a speedometer "permanently" mounted on the bike, or one with a heart monitor. Cadence is supposed to be between 85 and 105. I am NEVER at the 105 side of things. One coach I had also said, if you're below 80, you should be standing. And of course do what feels comfortable to you. Mashing is also harder on the knees than spinning.
Good luck!