If you have the opportunity and the resources, please go straight to Medicine. Medicine is a more difficult route in that their is no part time training and residency is a killer. As a MD you have all the options you want, just pay your dues up front (4 years college, 4 years Med school and residency/internship training). Nursing is more flexible in how you train, is a lower cost training and you can be an NP with a master's degree.
If you are doing well with the pre med classes, go through and get into med school. You also have to think about getting married and having babies. Once you have done med school, you will still have time for this. If you go the nursing route and then decide later to go to med school, you may not have the option with a husband and kids, bills etc.
My advice as a career nurse is go for medicine. You are young (youth fades quickly) and have brains-go for the prize. As a doctor, you call the shots and can practice as you like. As a nurse or a NP you will spend your career under someone's thumb.
As an MD you will have more credibility and if you wish to go into public health arena as a local health commissioner or health department official, you will have a greater chance of getting these jobs and advancing. An MD degree opens up so many doors while a nursing degree is vocational training and does not have the clout or prestige. I worked in a major teaching hospital that trained both MD and RN/NP's. The opportunity available to you and the places you can go are limitless with an MD. Go for the prize.