To reiterate what everyone above has been saying:
If you want to be close to the beach, then Florblanca would be the choice.
If you want to get away from everything and don't want to explore or have a small town feeling, then Punta Islita would be the choice.
If you want an array of restaurants, then Florblanca would be the choice.
Both are lovely, in their own way. Although I have not stayed at Punta Islita, I have passed through many times. The facility looks fabulous, but reminds me very much of an all-inclusive and I am not an AI type of traveler, so I would be very bored with the area by about day 3. If you are looking for an AI atmosphere, then this is the place to be.
I think the beach in Santa Teresa is much more appealing and the area has more to offer for activities and great international restaurant choices. Punta Islita only has a few choices for meals and if you are there for a week, the menus would become very mundane.
You have the choice to fly into both places and both facilities have ground transportation to and from the regional air strips (both landing strips are of equal distance from the hotel/resort).
Basically, it comes down to if you feel comfortable with being more secluded = Punta Islita, or if you want more choices = Florblanca.