I can't really think of any "cons" to a post-cruise Princess Tour. For myself, I would do the tour first and then relax on a Southbound cruise; this would shorten the flight home and probably save money too. But either way is wonderful.
Renting a car is a good way to see Alaska on your own. Personally, I'm not that adventurous anymore; but if you have teenagers, you are young enough to enjoy it.
It's against Cruise-Chat's policy to recommend travel agents; but if you click on this link to the Cruise Line Association, and then the "Vacation" tab on the left, it will help you locate a cruise/tour specialist.
Coral Princess is a good choice; it is a terrific ship, and of course Princess knows Alaska probably better than anybody.