The 7 day is the best way to begin, imho.....Just when you are getting into the groove on a shorter one, you have to get off.....most, and I am not saying it doesn't happen, but most who do shorter ones, regret that they did not do a 7...
Remember as well, that on most short cruises, the passenger age is generally lower, & the cruise a bit more boisterous.
Factor in the air if needed, and unless you get a deal, it seems a waste for s short one...Everyone who posted here, bets hubby will be hooked