Hello, fhrv!
Welcome to the Forum!

I have been searching between these two instrument as well. I am planning to buy a grand piano for practice.
Well, the main issue comparing these two grands is that both are made in the same factory. Yes, Bostons are made in KAwai factory in Japan on the order from Steinway. So, basically, both are of one and the same origin.
As for RX-3 - this is the grand I am planning to buy now. These days I've been playing it quite a lot. It is bigger then the Boston you mentioned, as you probably know - it is 186 sm, which is better if you search for the instrument for practice.
However, you should consider, that RX have rather deep bass, and if you plan to keep this grand at home practice room, you'll need a good piano tech to regulate it in accordance with your needs. Although, the same would refer to Boston you mentioned. Besides, it also depent on what level you are. Chosing between these instument prove you being a serious piano player, so I do not ask about it, since these instrument would be be good for advance pianist, or the one with serious intentions:)
As for price differences, during my own investigation I learned that since Bostons are made by the order of Steinway, which is a big name indeed, of course the price of the instrument rise, not because of the quality though, but due to the big name which is written at the courner of it. And, of course, with Boston, you'd pay also for the emotions you get from buying "the one from Steinways family", but mind, technically, it is not really so. The outcome is that, whith new grands, Boston is almost twice as expensive then Kawai, mainly because of the name written on it. Nothing to say that "Boston" has nothing to do with Boston (USA).
This price difference stays actual in your case as well. 1-year Kawai (supposdely kept in decent conditions) is basically a new Kawai. If you were considering a 7-8 year old KAwai, it would cost you much less then 14$K, not because of the quality, again, but name.
I want to mention again that Bostons are good instruments as well, but a bit over-priced, in my opinion.
If I could find a 1-year-old Kawai RX3 for such a price in my region (Russia), I would not think a minute! really, the only Kawai we have in here, is new RX-3 at 25,800$ (!) and RX7 at 44,000$.

Anyways, chosing the right grand for learning is a very personal and very serious thing. So, play both instruments, think your sensations over, and you will feel which grand is yours. It will tell you, really. And with such a purchase, the price difference is a very relative thing.
I wish you good luck!