Being a poor dead Zora musician > Being an uglier, much more evil version of Jaws. .... I love doing dolphin-breaches as a Zora...
The Boss Masks... are representative of great monsters that were holding the Giants prisoner. Your reward for defeating the bosses is the freedom of the Giants. They are not ordinary souls. They are god-type beings that needed to be freed from Evil, unlike the Deku, the Goron and the Zora, which were merely members of (supposedly) mortal races and were souls that partnered with you in the form of masks because they had unfinished business and needed your help.
Though, now that I think about it, if Link put on one of the boss-monster masks and became whatever GIANT he freed, that could be pretty cool - like a certain sequence in Shadow of the Colossus... Become a big, lumbering giant. Buuuuut, it would serve no purpose, really. Link, you only have how many hours to screw around now? Get back to hero-work!
The Fierce Diety mask, of course, is special, and requires sacrifice (of your other mildly-magical masks). And, still, he seems to be a "soul in need of help" even though he's a game-breaking warrior-god... but, again, a different entity than the imprisoned Giants.