I believe the OP was talking about "career changer" formal post bacc programs, not the "academic enhancement post bacc programs," which is for people that need to fix one's undergrad GPA. And I agree with the OP. It's different for people who came fresh out of high school and have the basic science and algebra fresh in their minds against those non-trads who are possibly changing their careers and are about to begin the daunting process.
OP, I think it really depends on what your undergrad major was. If you are coming from a relatively quantitative-heavy major (Math, Physics, Engineering, etc.), I personally think it won't be much of an issue to take classes with undergrads. Otherwise, if you graduated from humanities major, post bacc only classes might benefit you because 1. I believe these classes are not graded on a curve so it solely depends on the amount of effort you put into (correct me if I'm wrong). 2. Professors who have experience teaching this particular group of students are more accessible in terms of making the materials more comprehensible to the people coming from non-science background (whereas the same concepts might come really easy to those undergrad students who are already on a pre-med track).
I'm not an expert here but just throwing in my 2 cents. Good luck with your choice!