I would recommend the 3-year MArch, because it would save you two years of college. I heard that the 5 year B-Arch is really intense because it fits a 4 year BS Arch and a 2 year MArch in 5 years. Also a 5 year BArch might make you redo core classes if the ones from your BS in chemistry don't transfer. A 3 year MArch is basically a 2 year MArch with the pre-reqs for someone with no experience in architecture. It is made for someone like you to get into architecture without any prior experience. Personal I think a BArch would waste your time and be as stressful as a MArch, if not more. I have heard employers usually don't have a preference for a masters or bachelors as long as they are both accredited, professional degrees. Again a BArch is an accelerated BS and MArch combined. Since you already have an undergrad degree I would suggest the 3 year MArch, honesty I think that would the best option for you.

Also, it takes a long time to get licensed as an architect. You will need around 3740-5600 internship hours (depending on state), a professional degree, and pass 7 (will be 6 in a few months) exams to get licensed. Because of this I would recommend getting your professional degree as fast as possible, hence the 3 year MArch. However, if you feel more comfortable starting completely over with a BArch then go ahead. Do what feels most comfortable to you, just remember that additional experience and the exams will add about 2-5 years before you can get licensed. 

If you don't think you can handle the intensity of a program specifically designed for people like you, with no architecture background, then why are you even asking this question?

Do you really think you'll be better off in a program that is 1.5-2 years longer, in which you'll be the lone 28-year-old in a sea of teenagers, at the end of which all your peers and co-workers will question why you have two bachelor's degrees instead of a master's?

That you're even posting this speaks volumes about how you'll fare in any architecture program.