My H and I have taken many US trains cross country. As sometimes we have taken them separately I will combine our thoughts here. We love the trains...with bed, of course. It's a beautiful, relaxing way to cross the country and to get an idea of how beautiful, interesting and extensive the country is. I have not YET taken the train through Canada, but it, too, is on the list. Taking a train is about the journey. So here are our thoughts. Always get a bed via roomette or the bedroom. One ideal way to travel would be to get two roomettes across from each other so you have a view on both sides at all times. We have gotten roomettes each time and shared, but we get along well and haven't minded the upper bunk. It depends on one's size. At almost 70's and into 80's, respectively, we will either get two roomettes or a bedroom next time. You'll need to study what works for you.
From Boston, it is easy to take the Lake Shore Limited to Chicago, about 22 hours, overnight. It will arrive so that you can get on one of the cross country trains without an overnight stay in Chicago. My DH found the Southwest Chief the most interesting, traveling east to west, but it goes through the Sierra Nevadas, which he loved, not the Rockies. I traveled this line west to east so I missed the mountains. We found the scenery on the Zephyr to be beautiful through the mountains, through the plains, etc. We actually took the Zephyr both directions and felt both directions were equal. You get very varied terrain by taking any of these cross country trips.
DH thinks the scenery between the Zephyr and the California Starlight (along the west coast) to both be wonderful. But, they might have taken the Observation Car off the Starlight, so check that out if you decide to do the Southwest Chief (LA) and the Starlight to get to SF. DH says the Empire Builder was enjoyable, but it is not on the top of his list. If you want to go to Glacier National Park, it would be the train to take. He found the part through Oregon interesting, comparing the east and western part of that state.
There's a large component of people who love these trains. I think most of the naysayers for the US long distance trains are people who haven't spent any or much time on them. Oh, and the interesting people you meet on this end of the trains is wonderful!
Unfortunately, I can not yet compare these to Canada...someday though!