In the end I booked with DB. I am going up on Sunday via Germany (because it turns out the Metz - Luxembourg is a substitute bus). I am not in a hurry, and I imaging the Mannheim - Trier - Luxemboug will offer some scenery.
On the way back I book with DB as well, but I am going Luxembourg - Mulhouse - Basel - Interlaken, and DB managed to find the best tickets for me. TGV Inoui for Luxembourg - Mulhouse, TER Fluo for Mulhouse Basel, and a SBB supersaver ticket for Basel - Interlaken.
I must say, that the internation sales site for DB is getting ever better. It managed to book a trip in France for me that SNCF itself can’t…