I took the ferry along that same route in September 2011. I just showed up and bought a ticket.
I don't think there are any advantages to booking ahead of time. You might want to browse the websites of the two ferry companies that work that route to see if the prices vary based on time of day, or day of the week - but beyond that, there's not really much else to it.
I believe that the two companies might not be arms length from each other - I think there has been a bit of a drop-off of traffic on that route, and I think that they have partnered up with each other, but still operate under different names.
I took the 'Superfast' ferry, it was OK. Some of the ferries offer cabins, some offer cabins and dorms. If I had to do it all over again, I would probably take a dorm, rather than a cabin. The inside and outside cabins are identical in every respect, save for the missing window on the inside cabin.