  The stability and equation of state of quark matter are studied within both
two-flavor and (2+1)-flavor Nambu-Jona-Lasinio (NJL) models including the
vector interactions. With a free parameter $\alpha$, the Lagrangian is
constructed by two parts, the original NJL Lagrangian and the Fierz
transformation of it, as $L=(1-\alpha) L_{\rm{NJL}}+\alpha L_{\rm{Fierz}}$. We
find that there is a possibility for both $ud$ nonstrange and $uds$ strange
matter being absolute stable, depending on the interplay of the confinement
with quark vector interaction and the exchange interaction channels. The
calculated quark star properties can reconcile with the recently measured
masses and radii of PSR J0030+0451 and PSR J0740+6620, as well as the tidal
deformability of GW170817. Furthermore, the more strongly-interacting quark
matter in the nonstrange stars allows a stiffer equation of state and
consequently a higher maximum mass ($\sim2.7\, M_{\odot}$) than the strange
ones ($\sim2.1\, M_{\odot}$). The sound velocities in strange and nonstrange
quark star matter are briefly discussed compared to those of neutron star
matter.
