I take it you are comparing the CX750 to the AX760. IMO, the difference would only be noticed if pushing your machine hard, as in extreme overclocking where the 'cleaner' DC output would help some or if running in a 24hour setting where there may be a utility cost savings over time, other than that, put the £50 to a better graphics card (at least that's what I would consider)
"extra" features of the AX is higher efficiency and cleaner DC output
About the biggest difference is the manufacturer, the CX are made by ChannelWell while the AX are made by SeaSonic (the AX760i is Flextronics but still an awesome PSU)