I leased a '07 Sport Trac V8 Limited 4x4 in August, up over 7000 miles now. The Sport Trac with independent rear suspension offers a smoother ride on rough roads than the Frontier, but that might not be as much of a concern in Florida. There is a bit more interior space in the ST, but the Frontier has a longer box. Performance wise, the trucks are similar, although you would expect the ST with a v8 and 6-speed to have a slight advantage when towing, while the Nissan offers about 1mpg better fuel economy. Both trucks offer quality sound systems, but the Ford ST offers more options.
For me, living in Canada where the frost gets in the ground and creates heaves in the road, the smooth suspension of the Sport Trac was key in my decision. Winter lasts several months, and I have experienced lots of bouncing around with my previous solid-axle equipped trucks. Also, I tow a 4400 lb, 27 foot travel trailer, and felt a v8 was better for the job. Plus, I have a Ford dealer in my little town, and have had good experiences with Fords in the past.
Get the Audiophile sound system if you get the Ford, it rocks. The tonneau cover, bed extender, and power rear window are also key options.
Best thing to do is drive them, and choose what is best for you!