If Northwestern is stronger academically overall than Georgia Tech, then that means the non-degree courses at NU would be more difficult. So it would be easier to keep your GPA higher at GT. I definitely wouldn’t want non-degree courses dragging my GPA down.
GT would be very close to home for the OP’s kid, and I’ll take the sunny South over the long winters and below zero wind chill factors at NU. If there’s not a substantial difference in cost, I would pick GT over NU in a heartbeat for BME.