Not equal, but similar enough. You can double spend for a while until no one trusts the currency or low confirmation numbers anymore.
Not at all similar. Outpacing the chain in POW is super linear in the number of blocks, that means the more blocks you need to produce, the higher the cost to you and this relationship is a curve which curves upwards towards 'very high cost'.
Once you own some masternodes, the cost is zero for the attacker.
Similar in being able to double spend only a short while until people notice what's up and dump their coins and/or stop accepting them as a payment. Does the superlinearity effect get to matter enough to offset the fact that if you crash the currency your masternode coins are now worth zero but you could still sell the mining facilities and hardware and recoup some of the cost?
Just as in the case that asymptotic computational complexity models
don't guarantee that there aren't real world scenarios that deviate from the asymptotic case, the same applies in this case.
You can paint scenarios where it seems PoS and PoW both have risks. But the point of the asymptotic analysis is that at the extreme, mathematically PoS can't be persmissionless but in theory PoW could be if you can find a way to squelch the power of the longest chain to blacklist the minority. Even if you can't squelch that longest chain power, it remains true that theoretically the attacker of the PoW chain must continue attacking forever at unbounded cost of electricity (and updated hardware), else eventually the control returns to the honest minority.
Squelching the power to blacklist in the short-term appears to maybe be a form of anti-aliasing. I need to better conceptualize and explain this.