First ,get your concepts clear, POS is not mining.
For 51% attack, if you read the URL topic, you will understand that. For 51% attack in POS, I need to hold 51% of coin. Should I attack the coin in which I am the majority holder. Ok Give me an example in which a POS coin suffered or threatened by 51% attack.
No, PoS is not mining. PoS is an consensus algorithm. But there is a process called mining.
You should read my post more carefully. Mining is achieved via staking (Makes sense, doesn't it ?).
Mining does not mean to dig bitcoins out. Mining is the process of creating a new block and adding it to the blockchain (irrelevant whether PoS or PoW).
Mining does (logically) work differently in PoS compared to PoW.
POS:
One who has large stash of the coin will get big share, making rich more richer.
The same applies to PoW:
One who has large stash of money will get a lot of asics (and therefore more hashrate), making rich more richer.
If coin forks itself and start using new algorithm then all the ASIC will be worthless.
Creating a hardfork is a senseless argument to bring.
The same can be applied to PoS: 'If coin forks itself and start using PoW then..'. Rubbish.
POS:
If coin holders want to keep somebody out of network they are able to do so by not distributing there coins outside their network.
Don't Mind, I am just saying if coin creator and his group of friend decided that they will keep all the supply and do transactions between them only , then no body else will get the coin.(Hypothetically possible).
This does only work if the 'coin creator' and his group do not share any coin from the beginning (i.e. from the first block). This would be similar to a private chain/fork.
A small stake outside of the group would enable people to generate more 'outside of this group'. How is this either a pro/con regarding PoS ?
If it is a 'private chain': Who cares? Only the small group could care. Noone else would care about this coin.
If its not a 'private chain': It is not possible to achieve it in this case.
It is irrelevant.