In today’s world, the top 500 super computer altogether cannot 51% bitcoin network, But I doubt XPM hashrate will get as high as bitcoin or even 1/10 of it after releasing 5 years, ie 2018.
The key is how much hashrate OUTOF XPM network to 51% attack you. Moore Law also applies on the attackers hashrate. Without high hashrate, PoW is rubbish.
With regard to PoS, is the community a scarce resource? In a large cryptocommunity, I guess 20% people are hardcore cryptofans, and 80% are just fortune hunters, speculators, and the majority can easily shift to another community if they have not already joined multiple communities.