Mostar from Dubrovnik or Split?

How are you planning on traveling in between? If driving or taking the bus (and not stopping on an island en route), then I stay overnight. You'll get to see the evening light after day trippers depart.
Otherwise, take a peek at bus schedules to decide. https://www.buscroatia.com/ should help.

Thanks. We'll be spending a couple of days on Korcula and then traveling by ferry (or catamaran) to Split. So either way we'll either be on the bus, on a tour or in a rental car. And the travel time per rome2rio is similar, but I was just wondering if there is a "preferred" route. Also, is there a problem crossing the border in a rental car?

Since your itinerary is Dubrovnik to Korcula to Split, I think your best option would be a day tour from Dubrovnik. Trying to visit Mostar as a day trip by bus from any of those 3 locations would be difficult if not impossible due to bus schedules.

Good to know. Thanks for the advice.

If you do end up driving, you will need to check with your rental agency if your rental is approved for Bosnia. Sometimes there can be a wait at the border, probably worse during summer.
The Viamichelin website gives info on scenic drives. The Peljesac peninsula is especially scenic--I've been through on bus twice but will drive next time!

Thanks. We are planning on a day trip to the peninsula-maybe on the way to Korcula-we were thinking maybe we could get a driver who would drop us off at the ferry.