I'm building an £400 computer.
I've already got
*key board and mouse
spend more on the motherboardd and other components and buy an amd fx 6300, or spend less on a motherboard and components but buy an i5 3570k
you should be asking AMD FX-8350 or 3570k. they are closer in price and performance. What do you plan on using the pc for? if you plan on gaming go with the intel if not flip a coin. The Amd fx-8150 is not a bad alternative either.