Hi frankenstein,
I think only difference between P&S with the same digits/letters is parts qualification process. Of course this has to be confirmed by NXP guys, but I think that PPC parts doesn't pass all needed tests, which SPC parts are passing during production phase. From software point of view both devices behave in the same way (tested on PPC & SPC devices with the 1N81M mask). 'MPC' is family signature (i.e. both 'SPCxxxx' and 'PPCxxxx' devices are 'MPCxxxx').
Regarding 'H', 'G', 'S', this is feature set available. I'm working with SPC5748GG*, this is what we required and requested. according to the datasheet, 'H' is the superset of 'G' and 'S' options, but wasn't available when we purchased our initial lot of MPC5748 devices.