...it's probably not in the way you think it is.
Common media mythology states over and over that muscle is more metabolically active, so if you have more muscle on your frame, you're burning more calories just sitting around. This is true, BUT the difference is practically negligible. Plus, fat is pretty metabolically active, it doesn't just sit there like dead weight.
I'm only going to go through a brief overview of this, but the science is out there.
If you read Gary Taubes's "Why We Get Fat", you'll realize it's NOT about calories in and calories out and you'll also learn another neat little tidbit: you don't get fat because you over eat, you over eat because you are fat. (Of course, there are the fun little addictive tendencies due to sugar and personality, but that's a whole 'nother can of worms.)
Fat cells want to hold onto their stores, and they want to greedily take in more. When supplies get low, they will release hormones that tell your brain you are starving, to eat more carbs and create more insulin to get more into your fat cells. So fat isn't just sitting there, it is actively creating and releasing hormones that will make you want to eat more, and want to eat more CRAP.
That's why for some exercise won't do much except make people eat more, unless it's accompanied by a good (read, something not full of carbs) diet.
So, okay, resting muscle and fat doesn't have that big of a metabolic difference. Why is having more muscle good for your metabolism?
Usually because the activity done to attain that muscle is what is fueling your metabolism.
To put on muscle, you need progressive fatigue and overload. Heavy weights and high intensity training (think about how sprinters look) ups your post workout metabolism for up to 48 hours, depending on the level of your intensity. (Look up studies about "EPOC" to read more on this.) Your fat isn't going to be more metabolically active after a training session, just the muscle. Now, two days later and you've only been resting, the metabolic difference is going to go right back to negligible.
So to sum up: Same song and dance. Train hard, lift heavy, run fast. And before anything else, EAT RIGHT!