a lot also depends upon what you are looking for in your cruise experience.
Holland America is much more of a "traditional" old fashioned-for lack of a better word, cruise experience. They don't have all the "bells and whistles" that some of the more mainstream lines would have.
Princess would be a bit more livelier with more daytime and nightime activites.
You want to go in June and you haven't booked yet? Better check into it today! Cruises fill up pretty fast anymore, at least on the two lines we sail, RCI and Celebrity.
I don't know about the other lines, but we sailed Summit in 8/03 and really enjoyed it--cruise and land tour.