From the manufacturer:
Boston uprights offer the same standards of excellence which characterize all instruments designed by STEINWAY & SONS. A Boston upright will give you the same opportunities to express your musical skill as a grand.
In comparison to other pianos, the Boston has less string tension. This reduced string tension allows for a larger, tapered soundboard, creating longer sustain, and more singing quality in the tone (as well as longer piano life). A wealth of other engineering enhancements, including optimal placement of ribs, braces, and bridges, also contribute to the Boston’s superior tone and greater stability.
Each Boston piano soundboard is crafted of Sitka spruce, long proven to be the most resonant material available. Boston soundboards are also precisely tapered, which allows them to vibrate more freely. In conjunction with a number of special technologies — unique patents of STEINWAY & SONS — the result is a powerful, sustained tone.