In that case, it will cost the "government" much less to destroy a PoW chain. Also it might not even be possible to buy up 51% in a PoS chain if the coin is simply not for sale. But it's ALWAYS possible to get 51% in a PoW chain.
Too quick on the reply - the "resource needed to gain >50% is ASIC" and they are in short supply - so a government would have to set up manufacturing at least as good as the world's best (and keep it secret) to do so.
Of course they could just try and "purchase" existing ASIC so that would depend upon the pools as to how easily they can be *bought out* (and whether people notice this going on).