So which opportunity will give you the best chance for doing what you want to do in the future once you are done with the PhD?
For your RA, what are you specifically studying in each lab? Does the idea of studying that topic excite you?
At any of the schools, would you also be required to be a TA for any courses? If so, are they courses you are interested in?
Are you contemplating a career in academia after obtaining your PhD? If so, consider how well respected the mentor you would be working with is. Are they renowned in their field or just starting out? How many PhDs have they mentored, and what are they doing now?