PoS(hit) can never be secure, because if it has a functioning markets (which it must in order to be widely adopted and liquid), then one can borrow stake, attack the coin (which requires much less than 51% to for example delay transactions by some N blocks where N is a function of percentage of coin supply held), and then pay back the borrowed coin with cheaply bought coin as the price collapses due to attacks. You could simultaneously short it (i.e. which you did when you borrowed the coins, but sell some for fiat before you attack) for profits. Alternatively borrow fiat (or other cryptocoin), buy stake and short to profit and pay back loan. Also PoS can't distribute new coins, thus eventually the coin supply shrinks asymptotically to 0.

With PoW, your borrowed mining hashrate would eventually reach end of contract and the coin would repair itself. And you'd need much closer to 51% to do damage. You would hope to be able to purchase the coin at cheap prices, wait for it to rise back up and then sell it for fiat to pay back your loan. Much less plausible.

However if you are up against the corrupt State that charges cost of PoW mining to the collective, then we're screwed with profitable PoW also, except I have the idea to use the unprofitable PoW of every person's computer in the world (with latency preventing them from farming out to ASIC), which seems might be even too much of an expense for China to hide the subsidization of.