Yes, the two of us shared a 4-berth cabin (despite Martin snoring like a chain-saw). I'm not sure how much extra the cabin was, but having a shower and climbing into clean sheets after our very hot* ride across Greece was worth the extra.
From what I could see, the deck camping was limited to a number of motorhomes.
You ought to carefully check sailing times and prices. We used Anek Lines, but SuperFast ply the same route.
*We rode across Greece with temperatures of about 35°C. I have never ever seen my bike's oil temperatures rise to 6 bars when riding at 80-ish mph
Greg