Where to stay in Germany as a home base:

St Goar will also be a good home base.

Bacharach is very charming but very small and not convenient as a home base.

I've used Bacharach as a base and it had been fine.

mary jane,

I also used Bacharach for a home base for touring both the Rhine and Mosel, and found it to be very convenient. It is a small town, but that's part of the "charm".

I have been to St. Goar twice this past month and I don't know what is going on here, but the place is isn't very inviting right now. Still major road construction going on between the town and the river. Half the stores and restaurants are either closed or for sale. It just wasn't very interesting walking through here and I wouldn't advise an overnight here right now. Atmosphere is totally lacking. Ober-Wesel, Bacharach, or Boppard are going to be better than St. Goar.

Actually, Koblenz has a lot of sightseeing possibilities and this might be your best base for seeing both the Rhine and the Mosel with the least amount of waiting on trains going in the proper direction, and this includes going up the right side of the Rhine as well as the left.
http://www.koblenz.de/stadtleben_kultur/koblenz_allgemeine_infos_e.html

I'm hoping this post is still lively enough that I might question the experts about a similar predicament.
I'm planning a visit to the Rhine/Mosel area for August 20 through August 27 (About 8 days but flexible -- could be 1-2 days longer or shorter).
I was thinking of basing in Bacharach for 5 nights and then switching to Cochem for another 3 nights. We have a car. 8 nights total might seem like more than necessary but we have lots of time and like to relax and go slow.
One criteria that might help to select a base or bases is hotel class. We're a bit spoiled and really prefer 4* or very good 3* accommodations.
Should we base in Bacharach? Cochem? St Goar?

mary jane, you've posted under 'Rick Steves Tours'; are you visiting the river valleys in conjunction with one of his tours? That information will make a difference in our suggestions...