Southern Florida is too broad a term for anyone to give you really good advice. Have a look at some service maps. That should tell you pretty quickly what to go with. Also, keep in mind Tmobile doesn't force you into a contract while sprint will. Assuming you have a handset currently which you could get unlocked, you can try tmobile out for as cheap as $30 for the month. This won't be unlimited data, but you will be able to Gaige whether reception is adequate for you or not and then upgrade to another plan.
so im leaving att because im being kicked off my moms family plan (growing up sucks)