In my experience with mining coins and 51% attacks, the coins that have low hash power get attacked the easiest because all it takes is one powerful miner, or a group with the same intentions.  

I don't know much about POS, but i'd assume as long as you have a running node you're contributing towards the network power.  Even locked; correct me if i'm wrong.  Then again I guess it all depends on the algorithm/client side application.

I'm wondering how coins like OKcash and nxt solved this wallet staking without leaving the network vulnerable, those clients have to be unlocked to stake.  Never made much with them.  Potcoin there is no incentive to keep the node running, thats what I'm saying could be considered an issue.  Holding potcoin in the QT wallet is guaranteed residuals as long as we make it through this bear market.  

Break or bust by the EOY