Although they were outside cabins, we traveled on the overnight ferry so the window did not make a lot of difference for most of the trip. The photo here of the AA2** 2 BED OUTSIDE CABIN + WC + SHOWER is pretty accurate except we had 4 berths in each cabin:
http://www.greekferries.gr/bsf-en/bl...comodation.htm
The cabins were very narrow.

The toilets were not great overall, and those outside the cabins quickly devolved into a wet, messy, 'bring your own toilet paper' situation, so if the extra money is not a big consideration then it might be worth it just to have your own facilities.