  The degeneracy between disc and halo contributions in spiral galaxy rotation
curves makes it difficult to obtain a full understanding of the distribution of
baryons and dark matter in disc galaxies like our own Milky Way. Using mock
data, we study how constraints on dark matter profiles obtained from
kinematics, strong lensing, or a combination of the two are affected by
assumptions about the halo model. We compare four different models: spherical
isothermal and Navarro-Frenk-White halos, along with spherical and elliptical
Burkert halos. For both kinematics and lensing we find examples where different
models fit the data well but give enclosed masses that are inconsistent with
the true (i.e., input) values. This is especially notable when the input and
fit models differ in having cored or cuspy profiles (such as fitting an NFW
model when the underlying dark matter distribution follows a different
profile). We find that mass biases are more pronounced with lensing than with
kinematics, and using both methods can help reduce the bias and provide
stronger constraints on the dark matter distributions.
