From what IRC Techspeed telling me, its to do with the weather seals around the calliper pistons. The MG ones have them, the 'AP' ones don’t, or they have them and they are a different spec, cant remember exactly it was a while ago when we spoke about it. From what I understand it means that the APs are susceptible to salt ingress into the pistons and the corrosion associated with it. This is not an issue on a race car where the callipers are stripped and rebuilt on a regular basis, but on a road car it could cause some problems if you don’t look after them.
AFAIAA they take the same pads. ATM I can't see an upside to to the AP ones other than if you specifically want blue ones.
With sets of MG callipers,disks & pads selling on ebay for less than the cost of a set of new AP discs form X-Part, its a no-brainer to me to go for the MG ones.