We've done two: a 7 day out of Vancouver that returned to Vancouver, and a 7 day going north that we flew back from after an additional land tour. One was Holland America and one was Royal Caribbean.
For us, the cruising without a land portion was unsatisfactory; we felt we were missing most of what we went to Alaska to see.
Having done the land tour, I am absolutely convinced that the cruise alone is far less satisfying, and I am equally convinced that a land tour is ridiculously expensive and affords nothing more than renting a car, driving, and making a few hotel reservations would. After all, it is the U.S.; roads are just like lower 48 roads, and the people speak English. I would have preferred renting a car on the last one over booking the tour, but the couple we were going with absolutely wanted the tour. Afterwards, they said, "That was silly; we should have rented a car."
As to the difference in the two companies, the Holland America ship was smaller, so the entertainment options were MUCH more limited--instead of "shows" after dinner, they had game show type entertainment most of the nights, and I'm not a fan of that sort of thing.
I would suggest booking mid-summer, even though prices are significantly higher then. Weather is a huge factor, and your chances of good weather are significantly increased in July. We did the 7 day Holland American return to Vancouver trip on the last sailing in September--there's a reason prices are lower then. We did the 7 day cruise and land tour in July, and were very pleased--much more chance to see glaciers calving if the weather is warmer, too.