  We map the likelihood of GW190521, the heaviest detected binary black hole
(BBH) merger, by sampling under different mass and spin priors designed to be
uninformative. We find that a source-frame total mass of $\sim$$150 M_{\odot}$
is consistently supported, but posteriors in mass ratio and spin depend
critically on the choice of priors. We confirm that the likelihood has a
multi-modal structure with peaks in regions of mass ratio representing very
different astrophysical scenarios. The unequal-mass region ($m_2 / m_1 < 0.3$)
has an average likelihood $\sim$$e^6$ times larger than the equal-mass region
and a maximum likelihood $\sim$$e^2$ larger. Using ensembles of samples across
priors, we examine the implications of qualitatively different BBH sources that
fit the data. We find that the equal-mass solution has poorly constrained spins
and at least one black hole mass that is difficult to form via stellar collapse
due to (pulsational) pair instability. The unequal-mass solution can avoid this
mass gap entirely but requires a negative effective spin and a precessing
primary. Both of these scenarios are more easily produced by dynamical
formation channels than field binary co-evolution. Drawing representative
samples from each region of the likelihood map, we find a sensitive comoving
volume-time $\mathcal{O}(10)$ times larger in the mass gap region than the
gap-avoiding region. Accounting for this distance effect, the likelihood still
reverses the advantage to favor the gap-avoiding scenario by a factor of
$\mathcal{O}(100)$ before including mass and spin priors. Posterior samplers
can be driven away from this high-likelihood region by common prior choices
meant to be uninformative, making GW190521 parameter inference sensitive to the
choice of mass and spin priors. This may be a generic issue for similarly heavy
events given current detector sensitivity and waveform degeneracies.
