Personally, I think Maul did more with his less time than Grievous did with all of his.
I'm sure a lot of people here would agree with you. Maul's stock has always been high. He's the 'Boba Fett' of the PT... little screen time, huge fanbase... and a monster following before the film even opened.
I would agree that he's more popular than Grievous. He's more of a bad ass. Kids can act like him easier. He has cool moves. And skills. He has 'skills' and chicks dig that.