The Empire Builder is one of my "home" trains, I use it fairly regularly to head to the east. I also have ridden the Canadian regularly for the last few years. There is no question in my mind that the Canadian is the superior experience. And ride it eastbound.
You spend most of the day in, or close to, the mountains the first full day, as opposed to the Empire Builder where you are done with the mountains by 10:30 am or so.
The prairies through either Saskatchewan and Manitoba or eastern Montana and North Dakota are pretty similar, though I find the long stretches of tangent looking forward in the dome to the next set of grain elevators marking the next little town on kind of hypnotizing.
However, with that said, going as far as Winnipeg gives you the real experience and is a long enough train ride (2 nights). The last full day through boggy northern Ontario is no great shakes. I just like being on that train, so I am ok with miles of swamp.
Be sure and keep an eye on Via "Sleeper Plus" deals page,
http://viarail.ca/en/deals. For selected departures, you can quite a good deal if you are flexible, the deals only come up 4 - 8 weeks (at most) in advance. Right now, for instance, you can score Toronto-Vancouver lower berth for the 10/12 departure for $1275 CAD.
Also, if you do go, be sure and sign up for Via Preference. One Toronto-Vancouver trip in a sleeper will give you enough points (spend of $999 CAD) to qualify for the elite (Privilege) tier, which gives you a 50% off coupon you can you use at any time. Note the 50% is computed on high season full fare, but it is still a good deal. It usually beats the deals page, and you can book it on any departure in the inventory that still has availability in the "discount" fare level, so it is not nearly as restrictive as the "deals" and you can plan ahead. Of course, it is kind of passenger railfan crack, as you have to ride at least once a year in order to keep the discount, so they've got me.
I take the train in the off-season, when it is a "normal" train 8 car train full of Canadians instead of the 26 car tourist monster it becomes in the summer. Plus you have all day access to the Park car as a Sleeper Plus passenger in the off season.