If i don't do Marksburg, is Braubach stop worth stopping at?
The castle tour takes an hour plus an hour's time walking up and back down plus whatever wait time you might have for your tour. Braubach is just a small tangle of streets at the foot of the cliffside below Marksburg, and it does not take much time to explore on foot. But it is in fact pretty spectacular in terms of the architecture and old-world feel you will walk away with. Except for the fact that Marksburg is in Braubach, you would not know anything about Braubach's charm if you are relying only on Rick Steves' publications. I invite you to take a look at the link below - the 3 pages of photos constitute a "spoiler" but I think if you go, you will still be charmed.
https://www.stadtbild-deutschland.org/forum/index.php?thread/7322-braubach-am-rhein-und-marksburg-galerie/
Braubach is no good for shopping, amusements, museums, or fancy dining. But for a quick look/feel for what a small town looked like around half a century ago, it's a good bet. Most tourists are distracted by cruises, souvenirs, etc. and don't bother with it - thank goodness, or it might be a very different place today. Map:
https://www.turismoindependente.com.br/wp-content/uploads/2011/10/braubach.jpg
Good German food in town (closed W & Th) : Zum Goldenen Schlüssel:
https://image.jimcdn.com/app/cms/image/transf/dimension=1070x10000:format=jpg/path/sc484ea64eed53c8f/image/i82e0948970fa8d90/version/1550765364/image.jpg
https://farm5.staticflickr.com/4143/4751924826_98980aa49f_o.jpg
So I'm not going to Rudesheim just to catch this ferry but using it as a way back to the west side after exploring.
Not visit Marksburg? It is in fact an entirely intact medieval Rhine castle that has never been destroyed by invaders or turned into a hotel or otherwise repurposed/abused. It is #1 on my own list of the top Rhine sights, and Braubach's one of the Rhine's coolest old towns. I can't imagine being in Koblenz and not making the 10-minute train ride to Braubach for both town and castle.
Besides Braubach, the other two old-world towns in the top 3 would be Bacharach and Oberwesel on the west bank, IMHO of course.
I'm not clear on why Rüdesheim seems important to you. My take... Rüdesheim is the "shiny object", the perfect Rhine town for shedding unneeded tourist Euros... jewelry, clothing, tourist restaurants, amusements. Aside from that, I do like Rüdesheim's gondola ride and the chairlift ride back down to Assmannshausen - the territory at the top is nice for hiking around - but I see the chairlift ride and the river gorge scenery above Boppard (which shares the west bank town with Bacharach and Oberwesel) as superior. And like Braubach, Boppard is only a few minutes away from Koblenz. That's why I suggested Braubach and Boppard on the same day. And if Rüdesheim isn't a must-see, it's certainly unnecessary to go to Rüdesheim just to get to the west bank. I'd organize this one day as follows:
- Morning train to Boppard - see town, do chairlift, take a walk for the views, have lunch at Gedeonseck terrace at the top of the chairlift perhaps? Awesome views.
- Cross the river. Use the Boppard ferry crossing. Runs all day long, every half hour on the half hour:
http://www.faehre-boppard.de/richtzeiten/
- From the dock on the other side, it's about a 10-minute walk into Filsen. Catch a train to Braubach there. There's a train at 13:46 that gets to Braubach in just 7 minutes. Another at 14:46.
- Take the 4 pm English tour of Marksburg. See the town
- Return to Koblenz whenever you are finished.
IME the east bank is a pleasant place to "explore" but with the exception of Braubach, most east-bank towns have less to offer than the west-bank towns, and I know your aim is to plan your visit efficiently - hence my suggested outline for the 2 days.