I agree, definitely go with school B. I'm in the middle of med school apps right now, and it seems that med schools care much more about your GPA than the institution it came from--provided it's a 4-year college as opposed to a CC. So, while a 3.5 from Harvard may beat a 3.5 from Podunk U, a 4.0 from Podunk will win out over a 3.5 from Harvard almost all the time.
If school B is "weaker," it should be easier too, which should help your GPA. And AMCAS is going to count all your old courses in your GPA, regardless of whether your second school gives you credit for them, so you might as well go to the place that will get you out in less time.
P.S. I have a similar question to Dr. Midlife's: if school B will give you 137 credits and uses the semester system, isn't that more than enough for a BA degree already? Or do you not have enough credits yet in your major?
At my undergrad school, you needed 36 semester courses (108 credits) to graduate, assuming you'd also met the requirements for your major and the "distribution requirements."