  A strong signature of a circumstellar disc around a high-mass protostar has
been inferred from high resolution methanol maser observations in NGC7538-IRS1
N. This interpretation has however been challenged with a bipolar outflow
proposed as an alternative explanation. We compare the two proposed scenarios
for best consistency with the observations. Using a newly developed formalism
we model the optical depth of the maser emission at each observed point in the
map and LOS velocity for the two scenarios. We find that if the emission is
symmetric around a central peak in both space and LOS velocity then it has to
arise from an edge-on disc in sufficiently fast differential rotation. Disc
models successfully fit ~100 independent measurement points in
position-velocity space with 4 free parameters to an overall accuracy of 3-4%.
Solutions for Keplerian rotation require a central mass of at least 4 solar
masses. Close to best-fitting models are obtained if Keplerian motion is
assumed around a central mass equaling ~30 solar masses as inferred from other
observations. In contrast we find that classical bipolar outflow models cannot
fit the data, although could be applicable in other sources. Our results
strongly favour the differentially rotating disc hypothesis to describe the
main feature of the 12.2 (and 6.7) GHz methanol maser emission in NGC7538 IRS1
N. Furthermore, for Keplerian rotation around a ~30 solar masses protostar we
predict the position and velocity at which tangentially amplified masers should
be detected in high dynamic range observations. [abridged]
