I have 4 days planned for Bacharach and the Mosel (Cochem) then onto Baden Baden. Is it best to use Bacharach as a base, or spend two nights in each...then onto baden?
It simply depends on what you want to do? Cochem has hourly train service in both directions, plus a handful of IC trains.
Yes, it depends on your goals. Bacharach to Trier takes close to 2.5 hours. Long journey there and back. If you want to visit Trier, you might use Cochem; If Trier isn't in the mix, then Bacharach is relatively convenient for seeing all the Rhine and the Mosel as far as Cochem (which is about 1.5 hours from Bacharach.) But Cochem does make your trip to Baden-Baden longer. I'd be tempted to spend 2 nights in Cochem first, then 2 nights in Bacharach to shorten the trip to Baden-Baden. (That said, I wouldn't be going to Baden-Baden unless I were a gambler or a spa nut.)