IMO, the old JEA southside generating station is no better than where it sits now (save the river view). It would be much more isolated, no transit connection, and no surrounding ammenities to offer. At least the Prime Osborne gives conventioneers quick access to the skyway and I95.
IMO, the "old" courthouse site is the best fit for a new center, assuming the skyway or some other form of transit can connect it ot the rest of the core.
Also (another pipe dream) if the transportation center were to be compacted into the existing convention centersite, a new center could be built where the proposed transportation center sits now allowing it the same easy access to transit and give the transportation center back its spine and become a truely effecient Union Terminal as it once was.