6 months later and having used both king keg and corny keg there is one major point to add:
with a corny keg comes the additional expense of a CO2 regulator but with this comes the capability to accurately control the pressure of CO2 on your beer and get the level of carbonation you want (something I struggled with previously).
It was this capability that made me upgrade and it's definitely worth the additional cost. I'll be getting rid of my king kegs in time but for now I'm using them to do a secondary fermentation before the corny. If I had it to do again I would not have gone the king keg route - but don't let that stop you buying mine off ebay ;)