On the grand scale Gjallarhorn and Aegis are pretty on par with each other. For big fights you can either have the Gjallar giving the mages enough MP to cure the PLD more... or the PLD can just be taking less damage as the result of the Aegis.
Aegis benefits mages nearly as much a Gjallarhorn in that regard. Hell, if the PLD doesn't go down, other people don't get their faces kicked in by some HNM as a result of a PLD that couldn't stay alive. Gjallarhorn does nearly nothing for melees.
Gjallarhorn is more useful than Aegis for something like Dynamis maybe, but in a good dynamis shell MP is never an issue anyway. I know in my shell our BLMs are all sitting at 300+MP virtually all the time.