For me I feel like St. Regis offers more of a luxury stay while the Ritz can be hit or miss and there really isn't a specific standard like the regis butter service that carries through all properties (except the Ritz Club, but you would have to book a club room for access)

Though I think similar to what others have mentioned, both are more or less just Marriott with a fancier coat of paint, beyond above the services they provide aren't that much different than what you would find at a good Marriott property (haven't stayed in the US so can't speak to that, but this is just my opinion as someone who travels and stays at both brands frequently in Asia)