My last job was with a company out at Cecil. Believe it or not, there's not much room left for airplanes there. All the existing hangar space is occupied, by a mix of private concerns, Customs P-3's, CG HITRON and some NG/ANG cats and dogs. JAX, by contrast, has a lot of room to grow and better infrastructure.
The city and Duval County seemed to be pushing Cecil less for strictly aviation business and more 'intermodal', as it has better proximity to I-10 and the railroads.
Embraer seems to like NE FL; they're building bizjets down in Melbourne as well.