MS is just a hundred quid or so, depending on if you buy a kit or a ready made one. advantages over L8 or P8 include being able to map it yourself, wideband lambda support with self learning for the fuel map using target AFR values, coil pack support, etc, etc.

as it is fully upgradeable, you can add extra features as you like as they are developed by members of the MS community.

if you want to know about it properly, read the MS forums