Exactly, Signal also has their own implementation for notifications. If you install it while no Play Services are present, it will fall back on this and works just fine (battery optimization must be disabled of course).
I use both Molly and Signal without play services and figured that from the same group I get notifications simultaneously.
Molly has some security advantages (like extra encryption, auto lock and ram shredding) and is available in the FDROID repository or on Accrescent. For most GOS users it's the obvious choice. But both are fine and I'd say if you depend on Google Play Store just use Signal, or if you want the fastest updates, online backups, donation badges or for some reason MobileCoin.
As you see, there's no general answer possible. Both are amazing for your privacy vs every other popular messenger and you can try both simultaneously if you're not sure yet.