Is there a difference between the grandiose delusions in mania and schizophrenia?

Yes: While people in severe manic episodes may become psychotic temporarily, schizophrenia involves a loss of contact with reality which may be prolonged without treatment.

No difference: No difference, but the associated symptoms are different.

Not in the delusions: Themselves, though schizophrenic delusions are sometimes more bizarre. The real difference is that the delusions of mania resolve with the resolution of the mania. Schizophrenic delusions do not usually resolve.

Delusions: Delusions in mania are usually grandiose involving themes of power & religious in nature. Delusions in schizophrenia are mostly bizarre & persecutory.