I'm not an expert by any means, but I would take 12 credit hours and ensure you have enough time and energy for everything on your plate, while maintaining your GPA. If you could keep your 4.0, I think that would be far more important in admissions than taking 12 credit hours for 2 semesters! You would be applying for the same cycle anyways, so you might as well take your time and ensure things go as close to your way as possible!