I could not yet test the sound of W. Steinberg's, because it was not tuned when I played, and sounded terrible. Dealer promised it will be tuned by Friday, so I will be able to test it.
Some people on this forum replied that W. Steinbergs are very loud uprights. I have not noticed it yet,
If the Steinberg sounds terrible, it's a blessing at least that it's not loud.
Both of these pianos are sub lines, which simply means they are not the line of pianos upon which their makers have built their reputations. They are built to fit into a lower price niche.
Yamaha, a Japanese company, builds the B3 in Indonesia. Steinberg, a traditional limited production German manufacturer, is having the AC series built in Cina. Stineberg's premium German line, the IQ series, features some very powerful pianos. The sound can get out of hand if the player does not have the skills to control it. I don't think that applies to the AC series though.
I wouldn't be concerned about the origin of either piano. Yamaha may have an edge in its product consistency and factory oversight, but the Chinese factory which builds the AC has built some very good pianos over the past few years It's probable that the Steinberg hasn't been prepped if it sounds bad. Pianos from China do require more effort from their dealers to bring them to their potential.
Do your A/B playing evaluation all over again when the dealer advises that the Steinberg is ready to go. If neither piano inspires, try out other brands in your price range. Don't get too distracted by deep discounts from the list price. It's generally more about a manufacturer's pricing strategy than a killer deal.