  The relativistic Sunyaev-Zel'dovich (SZ) effect can be used to measure
intracluster gas temperatures independently of X-ray spectroscopy. Here, we use
the large-volume FLAMINGO simulation suite to determine whether SZ $y$-weighted
temperatures lead to more accurate hydrostatic mass estimates in massive
($M_{\rm 500c} > 7.5\times 10^{14}\,{\rm M}_{\odot}$) clusters than when using
X-ray spectroscopic-like temperatures. We find this to be the case, on average.
The median bias in the SZ mass at redshift zero is $\left< b \right> \equiv
1-\left< M_{\rm 500c,hse}/M_{\rm 500c,true} \right> = -0.05 \pm 0.01$, over 4
times smaller in magnitude than the X-ray spectroscopic-like case, $\left< b
\right> = 0.22 \pm 0.01$. However, the scatter in the SZ bias, $\sigma_{b}
\approx 0.2$, is around 40 per cent larger than for the X-ray case. We show
that this difference is strongly affected by clusters with large pressure
fluctuations, as expected from shocks in ongoing mergers. Selecting the
clusters with the best-fitting generalized NFW pressure profiles, the median SZ
bias almost vanishes, $\left< b \right> = -0.009 \pm 0.005$, and the scatter is
halved to $\sigma_{b} \approx 0.1$. We study the origin of the SZ/X-ray
difference and find that, at $R_{\rm 500c}$ and in the outskirts, SZ weighted
gas better reflects the hot, hydrostatic atmosphere than the X-ray weighted
gas. The SZ/X-ray temperature ratio increases with radius, a result we find to
be insensitive to variations in baryonic physics, cosmology and numerical
resolution.
