I don't know OSGi very well, so take what I say with a big grain of salt.
Yes, the module system and OSGi have a lot of conceptual overlap. Both turn JARs into modules/bundles by giving them names and letting them define APIs and express dependencies. OSGi goes beyond that, though. It's service model is more refined and it allows bundles to appear and disappear at run time, something
Java's module system doesn't even dream of. If you use any of OSGi's more advanced features, there's a fair chance that the module system isn't powerful enough for you. Otherwise, I would prefer the module system because it is better integrated into the platform and tooling (although the latter may take a little more time to mature).
For what it's worth, Sander Mak, OSGi and JPMS expert, agrees - see
here.