Fetuin is a glycoprotein which has both NANA and NAGA. This standard has been studied well in our labs using the DMB assay and we have set acceptance criterion which is based on several years of historical data - this criterion is constantly being monitored and been updated as we collect more and more data about this standard.
GPEP is a glycopeptide with a biantennary disialylated glycan. This is a purified homogeneous glycopeptide that has been quantitated using qNMR. The acceptance criteria is set based on this qNMR value.
There are pros and cons to both standards. Fetuin is a glycoprotein which may match your own sample of interest better than a glycopeptide, however, the glycopeptide is quantified in a more robust way (since it is a smaller molecule that can be subject to NMR). Also, fetuin contains both sialic acids - which may be an important aspect for you, while, GPEP only has NANA. You will need to assess what is important for your assay - we always use both but this is a choice you will need to make for your specific situation.