After dozens of trips across the country in the past 38 years, I really can't say which direction is better for sightseeing - at least in May when there will be considerable sunlight. For the dark days of winter, there is a bit more daylight time in the mountains westbound.
When flying one-way, which my wife and I do often, I prefer to take the train westbound. For one thing, you will have the great mountain scenery to look forward to. The eastbound trip back has never been as exciting. Because VIA keeps a back-up train in Vancouver, the chances of being evicted from the train and put on a bus at Kamloops when the train is very late are now fairly slim. For several years VIA was rather naughty in discontinuing the Canadian westbound at Kamloops and eastbound at Capreol when the train was well behind schedule. But eastbound there is still a chance that you could be treated to a bus ride - even well into the night - if the Canadian is quite late arriving at Capreol. There is no hard and fast rule as to when VIA will short-turn the train at Capreol and when they will allow it to proceed to Toronto. I have known people who were bussed down from Capreol when the train was only six hours late and others who arrived in Toronto by train at 0700 the next day (11 hours late.) If you arrive quite late - say 0230 or earlier - you will normally be required to leave the train upon arrival. If later than that, you will probably be allowed to sleep on board (assuming you are travelling in sleeper) until about 0630.
Sometimes a make-up train is run from Toronto when the arrival is extremely late, but if there is insufficient equipment a shuttle train may take passengers up to Capreol. But it's the eastbound run that can be a real problem. Having said that, it appears VIA has not been using buses as much as they were a few years ago.
As far as peak season: the peak season consists start in late April and run until early November. The peak fares start on the 1st of June and run until 22nd October. VIA will run as many as 30 cars on the Canadian if the business is there. But if a particular run is lightly booked, they will shorten the train accordingly. I have seen or read consists of a variety of train lengths this spring: from 15 to 30 cars. This represents anywhere from six to 19 sleepers plus of course the Park car (a half sleeper.)
