I assume by BSP you mean bisulfite PCR and sequencing. If so, the advantages of this over MSP are:
A greater number of CpG residues analysed for methylation when compared with MSP (MSP is only one and that is identified with a methylation specific primer). Bisulfite sequencing looks at every CpG residue across the whole amplicon.
I would also say that primer optimisation is crucial for an MSP assay to work, it is not so crucial for BSP because you will be sequencing through the amplicon anyway.
MSP has it advantages in that it is far more quicker than BSP in terms of hands-on lab time.
hope this helps. I would be interested to see what other people think about this also.