I rode the Canadian annually for several years until COVID scuttled my last trip, as well as Amtrak LD trains. Often the Canadian one way, Amtrak the other, so my comparisons are the results of head to head experiences on the same trip.
1. OBS crews on average are better and more consistent on VIA. Yes, there are duds, but not as many.
2. Food in the diner was considerably better than Amtrak's "traditional" dining and the menu did rotate.
3. The roomette beds are considerably more comfortable on VIA, wider (at the head and shoulders) and an actual mattress.
4. For regular sleeper passengers (Sleeper Plus) booze is NOT free. It is free for Prestige, which is roughly 5 times more expensive than Sleeper Plus. There were welcome aboard receptions in the domes on departure from Vancouver, Jasper and Toronto where they serve cheap champagne, though.
5. Prices can be pretty close in the off season for a trip of comparable duration. Look for sales. If you become a member of VIA's frequent traveler program, you become an elite level member ("Prestige") with one Vancouver-Toronto trip in a private accommodation and get a coupon worth 50% off the undiscounted fare, which will be lower than most sale prices (40% off is the most I've seen in their sale page the last few years). I've ridden Vancouver-Toronto in a roomette for around $900 USD with that coupon and a good exchange rate.
VIA's price structure is much simpler. You have discount and undiscounted fare classes, both of which are visible on the website. The only yield management VIA does is control capacity offered in the discount class, and there are 3 seasonally based fare schedules. Peak, Shoulder and Off Peak.
6. While the Canadian Rockies are spectacular, the "good" parts are only a couple hours on both sides of Jasper. There's a LOT of prairie. I'd rate the Zephyr, Starlight and eastbound Cardinal above it for pure scenery.
But none of them have domes.
7. I much prefer the train in the off season. It is much more low key and enjoyable generally. Not to mention much lower prices and Park Car access.
8. In the off season, dome access isn't much of an issue, aside from the 4 hours around Jasper, and at departure. For that, just get up there early. I don't go up to the dome for the Rockies any more. I've seen it and will give someone else a chance. There are usually at least 3 domes. A Skyline for coach passenger, at least 1 Skyline in Sleeper Plus (unless it is a really short, minimum consist and light load, which I've never seen), and the Park. The Sleeper Plus Skyline isn't always staffed in the off season, though. The consist and staffing varies with demand and load.
The general ratio of non-revenue space, including the lounge areas of the Skylines, the Bullet Lounge in the Park as well as the domes, is higher than Amtrak. There's lots of places to hang out.
