How different is PoS and PoW in practice (for ASIC eligible blockchains)? I need to devote X units of currency to get Y hashrate to gain 51%. This is a stake,

How different is PoS and PoW in practice (for ASIC eligible blockchains)? I need to devote X units of currency to get Y hashrate to gain 51%. This is a stake, needed to achieve a certain level of PoW. One difference is PoS requires stake only from internal currency (you must buy/mine a stake in that currency), vs ASIC PoW you can use a foreign currency to buy your 'stake'. This might mean PoS is less volatile, or resistant to external attacks.