More powerful? AF, easily.
Rather have? Mjolnir.
Reason: Although the AF trumps Mjolnir in power, the nature of the power requires a very specific mindset and personality type. Not to mention that it's the source choosing who wields it and to some degree, how it's wielded. So it's more problematical to use because you have to take the source into account and use it the way it wants. Whereas Mjolnir has no such restrictions.