I think there is a substantial difference in the political tactics required, mostly founded in the awareness of those affected. It's easy to promise that the lowering of pensions is temporary until the crisis passes, while once you've taxed somebody's savings (rather than properly borrowing them, which is very common - pension funds are enormous holders of public debt), there's no cover to pretend you'll give it back.

EDIT: Also, public pension schemes can be allowed to become progressively underfunded to the point where it's impossible to fix and the responsible officials have long since retired. No so with private savings.