I personally prefer St. Thomas/St. Maarten. Both have great beaches (Magen's Bay on St. Thomas and Orient Beach on St. Maarten are my favorites). San Juan is historic and busy. You can also visit the Bacardi factory for a cool tour. Labadee has gorgeous beach areas and is more pristine than you will find in other ports. Truthfully, you can't go wrong with either of those itineraries!
You'd asked about Liberty being a stretched version of Mariner and the Voyager class ships - that is true. In fact many of the venues even have the same names. The Freedom class (which Liberty is a member of) has a longer, wider Royal Promenade than the Voyager class ships. The Promenade cabins are really nice as well - a GREAT aternative to both Inside and Ocean View. I would suggest if you can to get your cabin assigned on Deck 8 - the design of the window has it slanted on an angle, giving the feel of "more room" in the cabin. Also, avoid the Promenade cabins on Deck 6 as they are directly above the Royal Promenade, with nothing extra to buffer the sound.
Regarding the time of year - hurricane season - just remember that the cruise lines (all of them) will not jeopardize the safety of their passenger, nor will they be inclined to put their multi-million dollar vessels in harm's way. Hurricanes are easy to avoid, and in fact some of the roughest seas I have ever experienced (20 foot seas) occurred in January. I sail often, including during the months of August, September and October (peak hurricane months - highest occurrence being in September), and I have not run into any problems.
That's been my personal (and professional - I own a travel agency) experience. I think you chose wisely! I'm happy to answer any other quesions you may have as well! Have a BLAST!