Don't know enough about your situation to properly advise.
Helpful information would be:
reasons for wanting to go to graduate school (not the reasons you'll be telling interviewers)
other commitments (kids/wife)
financial situation (do you already have a decent job that is ok with you moonlighting with research?)
My gutshot reflex is to advise you to choose the path that advances your career the most rapidly. In other words, I'd advise against doing research "on the side." But that is my value system talking, not yours. Taking it slow may be the right choice for you. The research track is long though, and as a result, it strongly favors the person who does not hesitate and plots a straight course.