Now you gotta pick a downside. Dwarfed dick? Short sighted? Four fingers?
For Steel-Skin, I'd imagine that the longer I use it, the more fatigued I became and the longer I would spend on "downtime". Use it for a few hours straight? Spend the rest of the night practically useless for anything beyond being able to aim my pecker accurately when I piss. And maybe a bit groggy in the morning, but a good night's sleep would fix me.
For the sound one, to be honest I'm not sure. I guess just general fatigue like the Steel-Skin one, except not as serious since I imagine Sound Manipulation to put less of an obvious strain on the body then say, turning your flesh into literal steel for a lengthy period of time.
If using Sound Manipulation caused me to be extremely sensitive to sounds for a certain period of time afterwards, I would just stick with Steel Skin since that would be a devastating downside in a world that was in perpetual conflict. Imagine destroying countless zombies, and then five minutes later, ANOTHER horde showed up and you get incapacitated because all of that moaning turns into a nasty shock to your auditory system, which could at worst, leave you in physical shock and unable to move or in immeasurably painful agony. Or at best, excruciatingly nauseous and disoriented/confused, which if bad enough, would make you a serious liability in a battle. And I doubt ear plugs would be plentiful in a zombie apocalypse.
So, yeah, I would probably choose Sound Manipulation depending on how severe the after-effects were following prolonged usage. Sound could be one of the deadliest weapons. I mean imagine killing someone hundreds of feet/yards away without even needing to know where they are? Project a controlled sound wave and it could literally explode their brain inside their own skulls. The only possible downside would be friendly fire, unless some kind of system with you and your teammates was concocted. That or you learn how to control the direction and spatial width/length of said sound wave.
