I haven't cruised on the Spirit but have done several freestyle cruises and have done the So Caribbean about 3 or 4 times. I will say this right off the bat: my favorite itinerary in the So Caribbean and favorite islands are 3, Barbados, Aruba and Tortolla/Virgin Gorta.
Freestyle is not for everyone, if you have cruised on lines where pampering is the norm you may be a little disappointed. We have always found the service on NCL outstanding but freestyle does offer a little different type of service. Yiu do not have one wait person the entire cruise so you will not necessarily build up a relationsdhip with the waiters and waitresses. This does not mean the service isn't great. BTW you can request the same wait person the entire cruise, but this somewhat defeats the freestyle cruising philosophy or eat when and where you want. Our favorite part of freestyle is the many choices for eating. We enjoy Le Bistro as well as the other specialty eateries.
The choice to dress formal or go more casual on formal nights is another plus: some people think part of cruising is dressing to the nines and many will do just that, but equally as many prefer the casual dress code. It saves on lugguage if nothing else. We usually do a sorta inbetween. DH will wear a nice sports jacket with tie and dress shirt. I usually opt for a dressy sun dress, with or without heels and hose, depending on the wather.
The third advantage to freestyle focuses around the last morning of the cruise. Instead of gathering in some public place with carry on luggage and waiting for debarkation time you can stay in your cabin until the time to depart the ship.
Now for the So Caribbean, the islands are friendlier and most deverse than the eastern or western itineraries and the shopping much better. Virgin Gorta may be the loveliest place I have seen and Barbados offer so much to see and do. I don't know which So islands you will be visiting but all have something special. Dominica is breath taking and the people very friendly as well as St Kitts. I personally did not like St Lucia and may get off the ship if we were to cruise there again. It is the type of island you will love or hate. Hope this helps some, I know ohters that will have advise and opinions as well.