I like Bedroom A for solo travel because of the better soundproofing from neighbors. I noticed no extra noise from the door or wheels (but I was privy to the sound and drama of an air hose breaking then being repaired right outside my room). On the Coast Starlight and Texas Eagle, I had people walking past my room and never even noticed unless I had the curtain open and was looking that way. On the Empire Builder, I was at the very end of the train and had "private " access to the railfan window.
But noise from neighbors is a problem in Bedrooms, especially when one is traveling alone and not chatting with a companion. You hear every toilet flush, cough, sneeze, and muffled conversation from the neighboring room. This is because the sliding door which separates Bedrooms except for A is very thin, and is even known to slide open unexpectedly.
While Bedroom A is smaller, the actual reduction in square footage is minor. The main difference is that the space for the bathroom door to open is by the hallway instead of by the window, and having this extra floor space by the window combines with the floor space by the chair to make the room feel more spacious. I prefer Bedrooms other than A when I am traveling with a companion.