About the early/late riser issue: There's enough head room in the bunks of the cabin for 2 that it's quite comfortable to sit up and read in either one (while your mate is sleeping). Only the lower bunk has a view out the window, though. However it's easy to slip quietly out of the room and head to the park car or other dome car to enjoy the early morning scenery. Attendant generally sets out fresh coffee there around 6AM.
And to clarify Blackwolf's post: the "tight quarters" are in the bunk itself, if you try to fit two in one bunk. There's a fair amount of floor space in the cabin for 2, even with the bunks set up for sleeping. Toilet in the cabin for 2 is VERY small--something of an advantage if either of you have balance problems. My dad (with whom I took this train last summer) noted that it's pretty much impossible to fall in such a small space. ^_^ Shared toilets in the hall are quite spacious.
There's just about NO floor space in the cabin for 1 when it's in its sleeping set-up.
As noted above, there are pros and cons to each option. I recently booked tix for three elderly relatives and myself, for February, Toronto to Vancouver. Climbing up to an upper bunk was a decisive "no" for all three. Having a private bathroom was a big plus for my dad, with whom I'll share a cabin for 2. My other two travel companions chose cabins for 1. None of us thought the price difference (during a half-price sale) justified the berths' lack of privacy for daytime naps.