MacBook Pro 8.1 i5 2415M vs i5 2530UM processor?

...so I'm kinda confused as about to buy a MacBook Pro 8.1 13" with the i5 2.3Ghz processor which costs around £1000. Thing is for the same money Curry's seem to sell the same MacBook Pro 8.1 for the same money with an upgrade of the hard disk from 320Gb to 500Gb.

...seemed to be a non brainer to go for the version on sale at Curry's except for the fact that when I check the spec online everything is identical except the processor says it's an i5 2539UM as opposed to the i5 2415M expected....any ideas what the difference is if any....is this 2530 effort inferior and is it a better option to play safe and just go to the apple store?

...any advice welcome...this will be my first MacBook so want to make sure I get it right!