We did the bus up and the train down. We thought it was perfect for our party. Our bus driver/guide was so engaging and interesting on the way up. The bus stopped several times to allow us to get out and enjoy sites. So we were very active and involved as well. After all the activity going up, along with the meal and the dog cart run, we had enjoyed a really full day already. So on the train ride down, it didn't seem like too big of a deal if some in our party just relaxed and maybe even dozed off for a bit. The sites to see are basically the same going up as they are going down; they are just a slightly different perspective from the train versus the bus. But since we did the bus first, when we still had our morning energy, we were able to fully enjoy them at the stops. The train doesn't have those stops and the extra personalization from a tour guide. So it is mostly on you the passenger to choose to stay alert and enjoy the sites or to choose to take advantage of the rhythm of the train to just allow you to relax and enjoy some "quiet" time.
In the end, it may depend on your preferences and touring style. And, frankly, either way is likely to be enjoyable still.