My advice to people planning something like this is to avoid the Rocky Mountaineer and simply buy a normal train ticket. The RM is quite frankly a tourist trap rip-off in my opinion.

Personally, I would drive as you can stop and go as you please along the way. I would suggest checking the 'drop off charge' for a rental car from Vancouver and then compare that, the rental cost itself, fuel, meals, etc. to see what difference if any there is in overall cost if that is a consideration at all.