The biggest advantage to the Rocky Mountaineer is that it stops overnight and you sleep at a hotel, so you don't pass anything in the night. However, in June/July daylight hours are very long so not much is really "missed" on Via. If comparing Via sleeper cabins - which IMO should be viewed as the absolute minimum class of travel for this journey - to RM, the pricing is fairly similar. Via is $1200-1500 depending on the type of cabin and it looks like RM starts at $1500 (although that is for a seat with overnight in a hotel).