KA, I shoot and carry 47D's. I do have some Powermags, but I don't carry them because they have a huge billboard-like rollmark on the side that I don't like (open carry, on duty).
The Powermag has a steel follower that has to be more durable and longer lasting simply because it's steel and not plastic.
Of course, I'm not experiencing failures with my 47d's, so take that for what it's worth.
Both have well-made bodies and removeable baseplates. Neither has given me any types of hassle, regardless of shooting conditions.
The Cobra mag is certainly more expensive. The follower is a combination of plastic and steel, with the harder steel working the slide lock lever so there isn't any wear in the equation. Does it work significantly better to justify the higher cost? I can't say that it does. If my 47D's are functioning fine (I have to remember to get some new springs!), what can the Cobra do for me?
Honestly, after all is said and done, I would look around your house and figure out which brand is more available. If the CMC mags are in every shop, but you have to order 47's through the mail, go with the CMC and don't feel bad. Both are great if your gun likes them.