We are also looking into cruising AK in 2018 but will likely go with Princess for a couple of reasons:
1) They have itineraries that leave out of Seattle (where we're from, so no additional transportation)
2) Significantly less is $$. For 3 adults and 2 kids we would book adjoining mini-suites with balconies and for the 2 rooms it was still way less than the family verandah rooms on
DCL.
3) My kids will be 14 and 12--the last time we cruised DCL, they have had NO desire to be in the Edge club at all. They are happy hanging out at the pool or just exploring the ship with their parents (aren't we just so lucky). So no real need for the kids club on this cruise.
4) Glacier Bay!!
5) My parents are as high up in the Princess cruise tier as you can get (I've only cruised with them once); they'd likely go and we'd have them book all our rooms and we'd be under their reservation, giving us all the benefits that go with that (this is not a real reason to go with Princess over DCL as we'd book even without the folks going).
I just can't get past how much the AK cruises are on DCL! It makes those European cruises oh so much more tempting (and an easier argument to present to DH on why we should do Europe instead)!