Public corps and state corps upgrade to the same level.
The difference is not public vs. state.
The difference is country vs. enterprise.
A country controlled public upgrades to 200 like a state corporation does.
An enterprise controlled public upgrades to 225 like a private corporation does.
A Truly Public Corporation, in which all shareholders own less than 25% of total shares, upgrades to 250 regardless of whether it is country or enterprise controlled.
The only substantial advantage to making state corporations public, as far as I know, is that people can't attack them when at war with your country (keep plenty of anti-nuke missiles around though... the fallout forces corporations of all kinds to close).
The downside of making a state corporation public is that you can't infuse them with cash (which you usually should not have to) and that you lose some of the profit payments to shareholders.