For both scenery and atmosphere you're better off in Mittenwald. And it's far easier to get there and back as well.
http://requisitosparavisa.com/wp-content/uploads/2015/09/mittenwald_-3.jpg
Day trip guide to Mittenwald by Earl Steinbicker:
https://hastingshouse.typepad.com/hastings_house_us/travel_in_europe/
The Bayern Ticket day pass (€25) can be used to reach Rothenburg cheaply and return, but it takes 3+ hours each way by train, there are multiple changes of train and there's an after-9-am weekday restriction on the Bayern Ticket. Not good.
Mittenwald OTOH can be reached by DIRECT Regional Express train in 1.7 hours. Much more doable. And you can use the Regio-ticket Werdenfels day pass to get there from Munich, which costs €22 for the round trip AND can be used on any day at any hour. Buy it from a ticket machine. This railway map shows the route and the day pass coverage in red from Munich to Mittenwald.