We attended the famous Vis regatta, (in Croatian - Viska regata) last year and all I can say is; it was amazing !
Vis is, by my honest opinion, if not most beautiful, one of the most stunning Croatian islands.
We have found accommodation the moment we stepped from our boat.
There was an old lady with sign "zimmer - rooms" and we were showering in sweet stone made house in a second :)
The price of the apartment, if I remember well, was around 60 euros for 5 of us, I'd say it was rather cheap price if we consider the fact that we stayed only 2 days.
Apartment in Split had cost us around 100 euros per day, and it wasn't as near beautiful as this old stone made house with sea view, situated in the hill, just 200 meters from the center of Vis.
We are preparing for next Vis regatta right now as I write this letter :)
My vote goes for island Vis !