Krilo vs Jadrolinija catamarans from Split to Dubrovnik.

I don't think there is a ferry between Split and Dubrovnik, just catamarans. You also have the bus, about four hours (also scenic).
https://www.krilo.hr/
https://www.jadrolinija.hr/en/sailing-schedule/local-lines-2019
The primary difference is in schedules, so I'd choose the one you like better time-wise.

I would take the bus, as the scenery is better and there are far more options each day.

I would also take the bus or hire a driver. The cost could be the same if there are more than two of you. But, we have used Krilo from Split to Korcula. We haven’t used Jadrolinija for a few years. krilo is a newer company and from what we saw docked in the summer of 2018 their boats are more modern. Jadrolinija has been around forever (my husband and FIL worked for them is the 70’s) and their boats are older, many are car ferries. I would book whoever has a more convenient time and/or departure pier.

We are currently traveling in Croatia. We drove along the coast from Dubrovnik to Split. Scenery is so beautiful!
We took Jadrolinija catamaran from Split to Hvar Town on island of Hvar. One hour ride. Catamaran certainly looked modern to us. Very smooth & comfortable ride. No problem or complaints.

Based on suggestions, sounds like a bus transfer is not as dreadful as I feared. Weather permitting we plan on a day trip to Hvar so that would give us the water experience on the water. Thanks for your reply!

We took Krilo day before yesterday. It leaves at 0730 and arrives shortly after noon in Dubrovnik. The trip was comfortable and easy, not claustrophobic at all. You can walk around as much as you like and there is open deck seating as well as inside. We bought day-of tickets without a problem. It certainly wasn’t full. Costs more than the bus, but you don’t have to worry about tight quarters, leg room, or bathroom stops. Down sides: not much in the way of snacks/food and no WiFi.

We took the Jadrolinja catamaran in July and it was a long trip - 6 hours with stops for disembarking passengers in Hvar. Korcula and one other place. Not a lot of schedule options but it was more comfortable than a bus. I haven't taken the bus so I can't comment on that option. Which ever option you take, you have to book a few days in advance - at least in the summer.