Hello,
We have sailed Vancouver to Vancouver in May on Princess. The weather was beautiful! It was the first Alaska cruise of the season, and some man we heard in the elevator said that he had never been in May before, but in his prior 42 (!!) Alaska cruises, it rained some of the time, and on this one, we didn't have any rain. The only time it was really cold was the day we had in Glacier Bay.
We have also sailed on Holland America, twice, but not to Alaska. I think the two lines are very similar, and we like them both. Of course, our first cruise was on Holland America, so that will always have a special place in our hearts.
To me, it seems as if there is a wider age group on Princess, but we were in our 30s when on our first cruise, and ages were not a problem. We started cruising on Princess because they had itineraries we wanted to do, and they were a little less expensive than Holland Amreica.......I don't know if that is still the case. The cruise experience, to us, was quite similar.............we're not gourmets, but the food was similar to us, the activities were similar, and the atmosphere was similar........a bit more formal than RCCL. We haven't been on Carnival, so can't address that line, but RCCL has more physical fitness type activities than the other two, and less traditional activities, except for Bingo.........they ALL have Bingo!! 
Happy Cruising!
