To add, when deciding on Alaska, itinerary is very important. So once you've decided one way vs. roundtrip, then narrow down what's important to you.

All Alaska itineraries should include at least one glacier day, but the difference will be which one. For example, many itineraries only have Tracy Arm Fjord (some will list this as Sawyer Glaciers). There is gorgeous scenery for the length of the fjord (saw lots of wildlife as we sailed the fjord), but the glaciers themselves aren't that spectacular AND depending on time of year, ice bergs fill up the fjord for example, etc you may not be able to get all the way in to see the glaciers (or in the fjord at all). So if glacier viewing is a must, you really want an itinerary that has Glacier Bay or Hubbard Glacier.

Look at port times when considering itineraries. The longer you can get in most ports the better, especially in a port like Skagway, where you have the opportunity to drive up into Canada or take a ferry over to Haines, but only if you have a long period of time.

Once you've narrowed down basic itineraries, if you have more than once choice in lines after that, then look at the cruiseline atmosphere. Some are more formal, some are more laidback. Cruise critic can help you with this.

Consider what you are interested in seeing. Many of the options will be similar from port to port, but some experiences are only available in one place. So you'll need to prioritize what's most important to you (in both time and money). That being said, you need to be aware that Alaska's weather is highly variable. So for certain things, like flightseeing or whale watching, be aware that mother nature might cancel it.