Both methods are cost effective , as with POW you spend money on electricity costs in order to mine the block which could end up with an unsuccessful haul or successful. With POS it's the same except you use the funds in your possession which is on the line in contrast to electricity costs & mining equipment. As with the POW the more funds you use could result in a larger reward but also bares an increased risk of losing majority of your funds invested in the POS. Someone beautifully summed up POS, saying as it's basically putting your money where your mouth is.

I'm thinking to mine ethereum asap before the POS comes in , save up some coins until the protocol changes over. Then use the vast amount of eth you've stored to result in more using POS system. But also could the ethereum blockchain split again?, this time one POS and one POW! who knows :p