[MrWookie47]

You've gotta think about what hands your opponent could be playing, and what of those will call here. He could easily have Ax, and he'll call with it and you win. He might even have 3x, and might call with it and you win. He might have been betting a newfound flush draw holding one club, but now he won't call. There aren't really any hands that will just call the river, but beat you (AK is unlikely). He might have 5x, been betting his gutshot, and now you're boned. To me, the chance of that is small compared to the chances of getting called and winning. That makes betting out the best option.