The human penis was so perfectly created for sucking, it's literally the design-prototype for the popsicle. That's why it's so easy & straightforward. In fact, you can even do it while playing Mario Kart.
But cunnilingus requires a singular, uni-task commitment, cuz you're basically snacking on an open wound. There's no watching Netflix or doing ANYTHING else when you're going down on a woman: Your head is buried, like a frightened ostrich. If you're lucky, you can still hear the stereo. So trading one for the other is totally unfair for the guy!
I say, 3 fellatio = 1 cunnilingus. That's a fair trade. (I'm also open to nontraditional trades, like something Bobby Bonilla's agent would think of: 1 fellatio + 1 sandwich + 1 nap = 1 cunnilingus.)