Spectral can punish all clone cancels except for f4xx clone with ex soul charge. I'm not saying a variation switch is 100% necesssry but it's my preference. Ex soul blast can get out of corner traps across all variations. All variations can also punish any clone cancel except f4 clone with ex lift. Ermacs only safe wakeup is ex soul blast so if he has no meter, you get a free jump in into 50/50 off a hammer ender. Additionally if you have meter and read the ex soul blast wakeup you can ex hammer for full combo.
Spectral can ex soul charge through clones/punish most clone cancels (not f4 clone), ignore longer distance clones with ground pound and has a pretty nasty corner game in its own right. Ill take Cryo here so I can do great damage midscreen, armor through flight cancels with hammer and punish him hard for mistakes.
Mystic can ignore clones with choke and I believe can punish clone cancels with it as well. This can get rough for gm so Cryo is the easy choice IMO.
MoS I think could go either way but I personally prefer Cryo. On one hand you build better meter with GM and don't need it for damage so you can break his big damage more often. Plus as has been stated, Ermac doesn't really get to break out of the corner trap without ex soul blast and even then it's not exactly a get out of jail free card.
On the other, Cryo competes better in terms of damage off one touch, and can heavily punish the unsafe gods vortex and end in knockdown for a potential setup for 50/50 into more big damage.
From the perspective of an Ermac player I'm only scared of GM in the corner and with spectral's safety and hover, I'm not to concerned with him being able to get me there, whereas Cryo can do serious damage off one touch anywhere on screen.
Sorry for long post but these are my mains were talking about here