Disclaimer: I'm not a gung-ho Steinway worshiper that argues this and that of why they are the "best" piano in town.
If that RX-7 has the Millenium III action, it aught to have superb response. I have played M3 actions, and they are buttery smooth and uber responsive. Some RX series pianos I found to have a rather stiff touch, but these may not have had the newer actions installed. Then again touch weight is to taste, some like it stiff. You may hear other piano makers bash "plastic parts," but really in an action where dimensional tolerances and mechanical fluidity are the focus, "plastic parts" are to an advantage. They don't swell up with changes in humidity and have less potential to cause uneven response. To answer the question, who would want plastic in their piano: I sure do.
Steinway can be hit and miss. But when you come across a good Steinway, it is really really good. I don't care for the smaller model Steinway grands, but the model B speaks to me. If I were to get a Steinway, the model B would be my choice piano.
In your situation, I would lean more to the Steinway. If it is a good instrument, and it speaks to you, go for it. Saving $10k will not matter 40 years down the road, when you are still loving and playing your piano. Choose the piano that speaks to you.