For me, one of the most important considerations here is how well each powerset can be integrated into a relatively normal human existence. I think the Shazam powerset easily tops the list in that regard.
Another consideration is the possibility of an extended lifespan. You'd get one for sure as a Herald of Galactus, but at a heavy cost, because you really can't have anything resembling a normal life anymore, looking anything like Galactus' other Heralds.
You definitely don't get an extended lifespan with a GL ring.
You might get one with Mjolnir, if you stayed in your godly form all the time, but again, you can't really maintain anything resembling a normal life anymore, looking like a version of Thor. Even if you wear normal clothes (like Sigurd Jarlson), you'll still be an unsually tall, musclebound blond, with a warhammer that you can't be parted from for more than sixty seconds. I'm assuming that limitation would be in place, since it was for every other mortal wielder of Mjolnir.
I think you would get an extended lifespan with the Shazam powerset if you stay in your super powered form most of the time, and that powerset does allow you to maintain your normal appearance, more or less. You might end up more muscular than you were previously, which is unlikely to be a bad thing if you're male.
GL ring may be able to extend lifespan somewhat....didn't Guy give himself peak human strength with it?
As to herald, it may depend on what your base power is...if you could choose shape-shifting or something, maybe it would work...
But yeah, I want to have the power all the time, and be able to pass for a normal guy.