There are two tracks between Frankfurt airport and Cologne, the scenic one via Mainz/Koblenz and the fast one direct to Cologne.

The former is very scenic, a bit slower but also cheaper to ride! Wonderful views of the Rhine Valley - best to sit on the right if you are going from FRA to Cologne and on the left if you go the other way.

The latter route, which is often non-stop between Cologne and FRA is a dedicated high-speed track which basically follows the course of the motorway and goes hurtling through lots of tunnels on the way, so you don't really see anything. It is quick though!

If I were you I would consider to go up on the scenic route and back on the high-speed.