Several geometries have recently been proposed for spin-transfer oscillators, with the purpose of optimizing the output
power and the frequency dependence on the applied current, as well as minimizing the external magnetic field required
to stabilize magnetization dynamics. The two structures most compatible with applications involve hybrid multilayers,
including magnetic films magnetized in the plane, as well as perpendicular to the plane of the layers - alternatively
acting as polarizing layer for the current and as excited layer. Here, we present a quantitative numerical comparison
between the two geometries. We find that multilayers with perpendicularly magnetized polarizer and easy-plane
anisotropy have considerably better frequency tunability versus current and require lower threshold currents.
Nevertheless, steady state dynamics can only be excited from specific, field-dependent initial states, which complicates
the design of potential applications. Structures with an in-plane polarizer and perpendicular anisotropy free layer are
more reliable, as they are insensitive to the initial state. In exchange, devices based on this geometry require a small (but
finite) external magnetic field for operation.