  We estimate cluster masses and velocity dispersions for 123 clusters from
optical spectroscopy to compare the Sunyaev-Zeldovich (SZ) mass proxy and
dynamical masses. Our new survey, HeCS-SZ (Hectospec Cluster Survey of
SZ-selected clusters), includes 7,721 new or remeasured redshifts from
MMT/Hectospec observations of 24 SZ-selected clusters at redshifts
$z$=0.05-0.20 and not in previous surveys. We supplement the Hectospec data
with spectra from the Sloan Digital Sky Survey (SDSS) and cluster data from the
Cluster Infall Regions in SDSS (CIRS) project and the Hectospec Cluster Survey
(HeCS), our Hectospec survey of clusters selected by X-ray flux. We measure the
scaling relation between velocity dispersion and SZ mass estimates from the
integrated Compton parameter for an SZ complete sample of 83 clusters. The
observed relation agrees very well with a simple virial scaling from mass
(based on SZ) to velocity dispersion. The SZ mass estimates (calibrated with
hydrostatic X-ray mass estimates) are not significantly biased. Further, the
velocity dispersion of cluster galaxies is consistent with the expected
velocity dispersion of dark matter particles, indicating that galaxies are good
dynamical tracers (i.e., velocity bias is small). Significant mass bias in SZ
mass estimates could relieve tension between cosmological results from Planck
SZ cluster counts and Planck CMB data. However, the excellent agreement between
our measured velocity dispersions and those predicted from a virial scaling
relation suggests that any SZ mass bias is too small to reconcile SZ and CMB
results. In principle, SZ mass bias and velocity bias of galaxies could
conspire to yield good agreement, but the required velocity bias is
$\sigma_{galaxy}\approx 0.77\sigma_{DM}$, outside the range of plausible models
of velocity bias in the literature.
