The width of 520 sprockets is narrower. This allows the chain to also be narrower, which in turn alows the chain to be lighter. I've never heard of someone breaking a 520 chain with a liter bike, but all the liter bikes I've bothered to check do seem to come from the factory with 530 chains.

I'm inclined to believe, that in a market where saving as much weight as reasonably possible while keeping costs down is a big part of the game, the manufacturers would be using the cheaper and lighter 520 chains if they felt it was a good idea.

All other things being equal (riding style, power, maintenance, chain and sprocket quality) I believe that the 530 chain will last longer than the 520. I aso tend to believe that I'm not a fast enough rider to need this particular upgrade. I'll buy steel sprockets and factory sized chains until I need those particular upgrades to achieve faster lap times.