From my time reading SDN's Adcoms' advice to others, and anecdotes from other nontrad SDNers' postings.
Heavy course load makes sense if you are a traditional student. But you already have a college degree. Now, the purpose of taking postbacc science courses is for sGPA remediation. So taking a heavy course load only makes sense if you can be certain of A's and want to get into med school faster.
Remember, there are matriculates in their 30s, 40s, and even some 50s, so age is not a factor. SDN's mantra is that the journey to medicine is a marathon, not a sprint. Dropping to 12 or even 9 credits and getting an upward trend with A's will provide Adcoms with more assurance that you can handle the medical school workload. That is more important that taking 15+ credits. Getting a couple of B's in a postbacc, while not a killer, will not increase that sGPA and lower your cGPA to boot.
So take the more certain route to assure that your increase your sGPA.