Just my opinion
Keepintime : Is there any reason why you do not want to gain any additional muscle. You say you weigh 133 lbs. and you want to get to 120 lbs to eliminate that small gut you have. As far as I know, there is no such thing as spot reduction, meaning even if you do get to 120 lbs, it doesn't mean your gut will be gone. As Jenn stated muscle burns more calories, so the more muscle you have on your body means you would burn more calories, even when you're sitting down. I would suggest that you weight train your whole body and eat 5-6 healthy smaller meals a day And drink plenty of water. It doesn't mean you have to look like Arnold, but some weight training, a bit of cardio and a healthy diet is the way to get to your goal of 120 lbs AND lose your "gut". Of course that is just my thoughts. Anyone out there agree,disagree or can add on. Anyway, I hope you obtain your goals.