We also sailed on DCL out of Seattle. While I absolutley love Seattle, I'm looking forward to leaving out of Vancouver this summer on our Alaskan cruise. When you sail out of Seattle, the itinerary is different, not necessarily better or worse, just different. We had a very short stay in Ketchikan (we were sailing away from port while eating lunch). Then we had an even shorter (maybe 4 hours or so??) stop in Victoria around dinner time the next day. That was kind of an awkward stop due to the short amount of time and the time of day, but we did enjoy Victoria. We had late dinner seating, so we had a chance to get out and explore a little and really enjoyed it. When you sail out of Vancouver you miss the Victoria stop. One other difference, at least when we did it, was that instead of heading straight to the inside passage, we sped up the coast in open waters to Skagway, then headed back down through the inside passage. That first day we were really speeding along and the waters were rough - pools closed, some decks closed, etc. None of us get seasick and we loved it. Others, not so much.
At the end of the day, I don't think you can go wrong on any Alaska cruise. It's simply breathtaking. There are a lot of options: Glacier Bay vs Tracy Arm (though DCL only does Tracy Arm), one way cruises (again not an option on DCL) that allow for time in mainland Alaska vs a round trip, Seattle vs Vancouver. Honestly (and I understand this really doesn't help much!) they all are amazing. Our first Alaskan cruise was on DCL and we loved every minute of it. We are going on NCL this summer because we want a different experience (Glacier Bay and a one way cruise to Seward), but that does not mean that our DCL cruise was anything less than spectacular.
As for the verandah, yes they are wonderful for Alaska. We spent hours on ours. But make sure you budget for excursions, as there are some incredible options and they are very expensive. If you can't swing the excursions you want along with the verandah, personally I'd go for a less expensive stateroom. We went whale watching and to the Mendenhall glacier in Juneau, did the Takshanuk Mountain trail by 4X4 in Skagway, and took a float plane to Neets Bay for bear watching in Ketchikan. All were terrific. Also consider the budget if you plan on spending time in Vancouver (or Seattle if you decide to go that route) before or after your cruise.