I'm kind-of a newbie myself but have found that a combination of spinning and mashing has vastly improved my over-all performance. When I started cycling about a year ago I was 131 lbs standing almost 6' tall. Spinning was easier for me to maintain speeds and it seemed like I held a higher ave speed but as I got farther along and rode with some faster riders I knew I would need to improve my strength. I began to do intervals mashing it up the hills then spinning as smooth as possible down the hill. Now I'm weighing in at 147 lbs and yesterday I did 45 miles at 19.5 mph ave solo with no stops. Maybe you should just give both a try and see which works best for you.