Does a Monocot or Dicot have more endosperm?

The larger cotyledons of dicots store a larger portion of the total startup nutrient supply so there is less in the endosperm but dicot species with small cotyledons or a monocot with a single cotyledon store their nutrients primarily in the endosperm.  So the answer varies since cotyledons are part of the seeds food store while the endosperm/nutritive tissues stores the rest.

Most dicots, especially those with large cotyledons, absorb the endosperm's entire nutritive store as the embryo matures while still in the seed. The pair of cotyledons take up the space once occupied by the endosperm inside the seed coat so the nutritive balance has shifted from the endosperm to the cotyledon but it is not lost. It is still present awaiting germination and ready to support the first photosynthesis.

A monocot's single cotyledon takes a portion of the endosperm and a portion of the space inside the seed coat but the main nutritive function is performed by the endosperm.  So a monocot is likely to have more endosperm than a dicot but that does not mean the seed has a larger store of nutrients.