If I could sum up the difference of quality in one word I would use the word "consistency". The level of service on VIA was consistently at least "good" on my recent trip.
You can tell, from reading this forum that there are huge gaps in courtesy etc by Amtrak employees, one guy had a recent disaster trip to Oregon and back (different disasters in each direction). On the other hand, some of the best service I have received in my life has been on Amtrak. But I found the VIA experience to be smoother, more even flowing. The empoloyees were easier for me, a railfan, to strike up a conversation with. Perhaps they were less busy, as there are very long stetches in Canada without people or stops.
Check out some of the recent trip reports, from different people(two from me) about their Canadian experiences. Steve 4031 put out a good report. No one employee, for me, was just super, super super great but all were at least "good".
I suspect the old VIA equipment is kept in better condition, though the interior is all green and that can get a little dull after awhile.
Not sure about your question about how many nights. Most of the long distance trains in the U. S. are two nights, the Sunset would be an exception. VIA is three nights, for sure. Incidentally, "cross country" in railroad parlance, can begin in Chicago and go west, it does not have to begin on the east coast to be considered "transcontinental" in railroad accepted jargon. Same in Canada---a train does not have to begin in Quebec and go all the way to Vancouver to be considred "cross country". This may not be good geography but it is standard railroaders lingo from way back when.