Of course they're comparable, given that they were designed to fill the same role. I don't know precisely which is considered the more powerful, but
this guy's website is a good place to start.
It's important to consider his own criticisms of his methods, particularly that the gun power is calculated purely in terms of muzzle velocity with no reference to the energy retention of the round over distance, but from the second table, the MG131 is less powerful to the tune of 8 rifle calibre rounds per second as a very crude measure, in terms of cartridge power multiplied by rate of fire.
However in terms of efficiency, measured as (average round destructiveness score * rate of fire) / gun weight, the MG131 is considered more efficient in that it provides a greater weight of fire per kg of gun weight.
What you take from that is up to you, I just happened to remember seeing that website and finding it informative.
Essentially, the .50 cal Browning is considered by this particular fellow to be more powerful, but it comes at a cost of greater weight. Whether or not this information is perfectly accurate I have no idea, but the guy seems extremely interested in ammo and such, so he ought to have a vague idea at least.
EDIT: Haha, TOO SLOW, ME.