In my opinion I prefer POS. So many 51% attacks going on because of POW, the future is POS in my opinion and less energy consumption which is a good thing to all those not living in third world countries where electricity is cheap.
A 51% attack is quite possible with POS as well. Especially with low cap coins. A checkpoint server can help. But then the coin is basically centralized.
Even worse in PoS, not only can you do 51% attack by gathering majority of coins now, but gathering the majority of addresses used at any point in that altcoin's history. You can use addresses that are now empty to do a 51% attack in PoS. Just imagine people who don't have any of those coins anymore selling their old private keys to someone who is going to use them to reorganize a PoS "blockchain".