We have finalized our itinerary, thanks to this group! We will fly into FRA, go straight to Sankt Goar for two nights, three nights in Rothenburg ob ter Tauber, then to Regensburg to meet up with our friends . I’ve decided to get a rental car but we actually don’t need it until we leave Sankt Goar on a Sunday morning. Considering price and convenience, my question is if I should get car at FRA upon arrival or Sankt Goar area (note Sankt Goar would be on a Sunday morning)? Thanks!
It's not clear how you have arranged your time on the Rhine, so it's hard to say whether it might be useful during your stay. Assuming it isn't needed, as you say, then I'd consider visiting the east bank of the Saturday; Rüdesheim, Eltville and Braubach (Marksburg) are the usual destinations there. You'd cross the river by ferry in the morning, catch the train in St Goarshausen south to Eltville or R'heim, then a train to Wiesbaden, where a pick-up should be findable near the station. Of course, make sure you arrive prior to the Saturday closing time. The car might be useful after that for Marksburg Castle if that's on your tight agenda - save you a taxi ride or a walk up and back from the center of Braubach.
OR... see about a pick up in Koblenz at the Hbf station (I think Hertz is there) as it'd be on the way to Marksburg or to Mosel destinations or to Rhine destinations north of Koblenz. St Goar > Koblenz is a direct train ride.
I don't think Sundays will be productive pick-up days, except at FRA airport, perhaps.
With 3 nights in Rothenburg, and only Day 2 and part of Day 1 on the Rhine, I suspect you are depriving yourselves of valuable options on the Rhine; suggest you have a look at what else you might do on the Rhine for Sunday and postpone the drive to R'burg until late Sunday afternoon.
Earlier thread of OP with nearly the same question:
https://community.ricksteves.com/travel-forum/germany/fra-cologne-cathedral-and-rhine-valley
Friendly meant opinion: if you drive the way like you take this decision better use trains. Reason: road traffic in Germany can often be stressful, impatient and very fast-paced. An unclear way of decision taking and driving increases the risk of all traffic participants.
To your question: My answer stays the same.
One additional fact: renting a car at an airport or train station of a large city is always more expensive - it is a surcharge taken by every rental car company although they call the fee differently. It is approx. 10% of the daily rate plus VAT. So, at a longer rent it can sum up.
Again: check requirements for driving permission (not license only plus IDP?):
https://bmdv.bund.de/SharedDocs/EN/Articles/StV/Roadtraffic/validity-foreign-driving-licences-in-germany.html
Make yourself familiar with German road signs and traffic rules.
Frankly I'd add a day on the Rhine, and then take the train to Mainz or Frankfurt and get a car Monday morning. if you can't do that then get the car at FRA and plan to keep it until you fly back. Parking in Sankt Goar is limited, it's a small town.
There would be a better selection of vehicles and companies at the airport than there will be in a small town. As long as your lodging in Sant Goar offers parking, then that wouldn’t be an issue.
Where you’re returning the vehicle could be a determining factor. If the rental company in Sant Goar doesn’t allow drop off in a different location, that makes the decision easier.