I have both.
The King Keg is grand as long as you use 33 - 40% sugar/dextrose that you would use if you were bottling (any more is an over frothed nightmare).
It cost me 50euro for the tap and the kegs were cheap (about5?). The Gas bulbs are a little pricey. Its 7.99 for 5 (16g), I would generally use 2 bulbs to dispense a keg - that annoys me to be honest.
So you could do 2 kegs and 20 bottles for a typical 20l brew.
The Cornie setup is nice, the kegs are bigger at 20l, and the setup cost is considerable. I spent about 250euro. BUT the running costs of the gas are minimal - a 6kg tank from munster is 50euro and 10 to refill - yes that's 6000g !
The Cornie pint poured with the proper tank is something to behold - but I wouldnt shoot down the king keg - I cracked open a 5l keg on Christmas day with the brother - we had a blast -
oh did I mention - the cornies hold pressure so you can pour pints whenever you want - the 5l kegs have to be finished that night - its a law - they dont hold (well thats my excuse!).
So long term (higher set-up costs) -Cornie
Short Term (low budget) - King Keg.