My fitness-oriented (I won't say "obsessed" exactly, but getting up for the 5:30am "boot camp" class at his gym multiple times a week is certainly not sane!) coworker swears that if you build muscle, you will burn more calories when you are otherwise not exercising. Muscle requires more calories to maintain. When you do strictly cardio, you only burn calories when you're doing the cardio.
It's true that muscle weighs more than fat, and you might see some weight gain at the beginning, but the muscles will continue to work for you in the long-term.

Also, muscle doesn't squish like fat does, so you'll look better even at the same weight (or more!).
In particular, due to the prevalence of osteoporosis, I'd really love to see more ladies at the gym pumping iron! There isn't enough!
Speaking of which, today I deadlifted my bodyweight+10lbs.
