If I were you I would try Alaska for my first cruise for one huge reason. ADA. The USA is the only country in the world who has it, and while the US based cruise ships are now designed pretty much with disabilities in mind, and close to ADAAG requirements, (I saw minor non-compliance issues on the Princess Diamond when she was new 2 years ago, but I'm an architect with a specialty in ADA)the ports in other countries are definitely NOT accessible. Most of the ports of call for the Alaska cruise are on US soil, so they have to be accessible by law. I didn't get off the ship in Victoria so I can't say if that port has been adjusted, but my guess is that is has been.
As for the Alaskan port excursions, you can definitely take the tram to Mt Robert in Juneau, or the 1898 train up the mountain in Skagway, and there are lots of things and shopping right at the dock in Ketchikan. There is no tendering at the Alaskan ports, at least for Princess.
When we docked in Ketchikan, the ramp down from the Plaza level was not 1:12 so it was too steep for a wheelchair. I saw where there was another gangway coming from a much lower deck for wheelchairs to have access. Hopes this helps.