Hello, I'll be spending some time in Dubrovnik in May and would like to do a day trip to Mostar. I won't have a car. Is it worth spending the whole day in Mostar alone, in which case I might just take a bus there and see it solo, and perhaps take a guided walking tour there? Or is it worth it to take a private day trip with a tour company such as Trip Advisor or Viator? They pick up and drop off at hotel, which is convenient, but some reviewers said the tour didn't allow enough time in Mostar (3 hours) including taking time for lunch.
Depending on the bus schedules, would you be willing to consider spending the night there? That may be the most relaxing way to do it, assuming the bus trip is 3 hours or so each way (or more). You really don't need a guide in such a compact place except to get a good historical overview. For what it's worth, Mostar is very easy to do on your own. I highly recommend the Muslibegovic House Hotel, which is also a museum - it was a lovely stay (very atmospheric, interesting, great breakfast, etc.)
I went to Mostar from Sarajevo by bus, then to Dubrovnik by bus as well. I'm really glad I stayed overnight and would have liked to stay even longer, even though Mostar was quite small. It's a really pleasant place. Be sure to walk over on the other side of the bridge to explore.
The downside of the tours is, as you noted, a very whirlwind experience. If you go on your own, you will not travel as quickly/efficiently, but you'll have more time on the ground.
I took the day tour with Select Dubrovnik from Dubrovnik to Mostar in June, 2018. I chose that company specifically because they would allow me to take my luggage and stay in Mostar, not returning to Dubrovnik with the group. This tour picked me up at Pile Gate, made a 30" rest stop once over the border, then went on to Mostar. Our bus load (about 18) then was combined with another bus load for the walking tour of Mostar. Thus, we were some 32 people - way too many - trying to stay together, navigating the tight cobblestoned streets. The return trip was also going to stop at waterfalls on the way back to Dubrovnik, so I think that on this day trip, too, the folks returning to Dubrovnik were only going to get some 3 hours or so in Mostar. I would def look into bus schedules if you want a longer day. (rome2rio.com will show you which companies serve that route.) Rick's "Croatia" book gives good info about Mostar. You don't really need a walking tour; it's a small enough town (the historic part, where you want to be), and Rick's book gives good info about museums, coffee shops, restaurants. Take his advice about the place to eat on the river, Babilon restaurant. Fabulous fish dinner, fabulous views of the river and bridge. If you have the budget for it, I'd contact Ermin Elezovic (his contact info is in the book) or his wife (forgot her name; also in the book). They are magnificent people. Ermin - or a personable young man who works for him - can pick you up in Dub and bring you straight to Mostar. His wife does private walking tours of Mostar with individuals and small groups. I met her at their son's wonderful coffee shop (also in the book) and Ermin gave me a full day tour from Mostar to Sarajevo. Here's the link to my full trip report, with more info about Mostar somewhere in the middle: https://community.ricksteves.com/travel-forum/trip-reports/trip-report-solo-trip-to-croatia-bosnia-slovenia-part-1-split-croatia
Neither TripAdvisor nor Viator runs tours. They just list tours run by other companies. You might save money by booking a tour directly with the company operating it.
Personally, I'd opt for the bus, and I agree that staying one night sounds like a good plan.