I have read the same in the travel book we use; but the book we use is only updated every couple of years so maybe something has changed.
You are probably aware that there are two major carriers: Jadrolinija – they run the large car carrying ferries and Krilo – they run the faster catamarans. The websites don’t offer much help with the schedules IMO. The other thing to be aware of is if the seas are rough the catamarans may not run.
The only advice I can offer is my experience. I did this trip in September 2011 but spent 2 days in Korcula and a day in Hvar. I caught the ~10:30 AM Jadrolinija Split to Vela Luka then took a bus (1hr) to Korcula town (bus meets the ferry) arrived in Korcula town around 2:30PM.
At 6AM on the 3rd day I caught the Catamaran to Hvar – Buy your ticket the night before. They begin sales around 18 or 19:00. If the catamaran does not arrive due to weather they don’t sell tickets for the morning return. Spent a nice day in Hvar and then returned to Split on the next mornings catamaran – again buy your return ticket the day before.
Probably the best thing to do is once you are on the ground in Split is to go the ferry office down on the Riva and check the current schedules and options. If I have to choose between the two islands I think I liked Korcula better.
Best of luck and have fun.