bus or train from Trier to Frankfurt?

Is it interesting to be on the I-5?

Autobahns, like interstate highways, cut the flattest, straightest, most direct route between major points. You get a lot of trucks and buses in the slow lanes, and faster cars in the fast lanes. Sometimes there is construction or an accident and traffic slows down. You do get some interesting views but they are only for a few moments. Most of the time it is a concrete or asphalt strip with vegetation cut back a few metres then trees.

On the train you are much closer to scenic areas, still lots of trees, views of the river, you are somewhat closer to the vines of the Mosel, but the train, too, tries to be relatively straight and the Mosel, unlike the Rhine, is exceedingly winding whereas the Rhine is straight.

If you had a bus or car along the Mosel instead of up on the autobahn the scenery would be right there.

I don't ride buses so I can't compare the actual ride.

The train takes you to Frankfurt via Koblenz and Mainz and thus along Germany's most scenic train route through the Middle Rhine Valley. Before it reaches Koblenz, the train route follows the Mosel riverbank starting in Cochem; Cochem to Koblenz is also very scenic:
http://www.zughalt.de/wp-content/uploads/2011/10/DB11321.jpg

Trains are not subject to traffic conditions, so you are more likely to arrive as scheduled.

I recommend overnighting in the pleasant town of Mainz prior to your morning departure from FRA. Mainz is a very walkable town with a nice tangle of pedestrians-only streets that get you into the shopping and old-town zones. Mainz sends off frequent direct trains to FRA starting at around 4 a.m. - takes about 25 minutes to reach the terminal.