There's quite a large dispairty between the two CPUs.
Benchmarking the two against each other the 2520M has 7.5x a better AES benchmark score than its counterpart, meaning that under a stress test using a single core the i5 is extremely outperforming the Z8300. Even when utilising multiple cores using a multiple-core benchmark it has nearly double on the Z8300. On top of that, its also got 2.5x the onboard GPU clock speed over it aswell
A quad-core (Z8300) with such a weak load threshold isn't ideal to have, while generally if you want to play recent-release type games its advised to opt-out of dual-core CPUs as it just isn't enough anymore to provide performance in this day and age. In the case of osu!, while it isn't an extremely 3D-intensive game it may be a good idea to opt for the more recent i5-2520M so later down the line you don't have to continue forking out money to keep processors up to date. If in the case you actually DO want to play recent-type games, you'd have to go even higher to recent quad cores.