I favor neither version, but privatization is better than opt out. I have no faith that my state won't be stupid.
This tbh, enough states would be stupid or nihilist enough to go for the opt out
Privatization sounds like a nice and wonky idea on paper, but you'll have enormous transition issues which means either exploding the deficit or wanking current seniors. I'd favour (voluntary) partial privatization if it's done in a cautious way, maybe allow people to invest up to 15% of their SS taxes in personal accounts or so (if the budgetary situation allows for it, there will be some transition issues).
The second problem with privatization is that a lot of privatization proposals include some guarantee, which kinda defeats the idea of privatization and will inevitably end up badly for obvious reasons.