I'm not an aerobics fan. Diet's way more important than exercise for for most. But I'm hard pressed to think of someone whose done more to encourage people to like themselves and work to better themselves in my lifetime.
Diet is more important from the vantage point of raw calories. (i.e. In order to burn off a single jelly donut, you have to go running for something like an hour. So not eating the donut is probably the easier option.) But exercise is incredibly helpful in regulating hunger... thus making modification of diet easier. Plus exercise brings with it a whole host of other health benefits, many of which are directly helpful in weight loss.
You need a balance of both. I run for an hour every day so that later in the day, I can have a beer. The rest of my diet is pretty simple, eat what I need to eat and not what I want (which sucks because I love food). No snacks, no candies, no alcohol except for the beer I worked for earlier ... and that's just to *maintain* my current weight.
Seriously, being an old fuck really blows when it comes to weight gain; although I guess when I eventually get colon cancer it won't be as much of a problem.