We've done it both ways a couple of different times and I liked cruise first better.
--First cruise we took, we did WDW first and DH came down with a stomach bug the first night of the cruise, which he almost certainly caught at the parks. Ended up quarantined and missed out on the whole cruise. Ever since, when we've done WDW first I've been a nervous wreck and go sanitizer crazy worrying someone will get sick and we'll miss our cruise again.
--Second, I completely agree with what everyone is saying: the cruise is wayyyy more relaxing than the parks. But I don't understand why anyone would want to do the parks first as a result. When I get off my stressful job and travel somewhere on vacation, I love relaxing on the cruise, and then when I'm refreshed and have lots of energy, doing the parks. Otherwise I just go from tired (at the start of my vacation, from work) to suuuuuper tired (from parks), and am burnt out, achy and exhausted by the time of the cruise and don't enjoy it as much.
--Third, traveling home after a cruise is no fun. You are kicked off the ship early in the morning, bussed straight back to the airport, and often have a wasted day waiting around for your flight with nothing to do. At WDW, departure day is fun because you can do a big breakfast at the resort, swim in the pool, and even visit a park if your flight is later in the day.
That said, it is definitely more expensive to do the parks after cruise because you also have to book a pre-cruise hotel stay (some people fly in the day of the cruise, which would be wayyyyy too risky and stressful for me). So we still sometimes do parks first--I jut don't enjoy it as much.