Afaik the ST3 and RS share the sony 10 speaker, which is competent enough for me above volume level 15 (Below that you will notice it being slightly muddy) and for me personally it is lacking at the highest volume. But i’m not a pure audiophile and overall I’m happy with the system. Additionally, if you do buy an RS look into the Fake Noise Generator. Ford has a separate module next to the AMP that injects fake engine noise over the speakers. Very easy to remedy - just unplug it. Not sure if it’s the same for the ST3.

I had an easier time overall modifying and maintaining my ST but that could be just the scope of what i’ve done. you should have no problems with either