Hi! I am 3 months away from becoming a Family Nurse Practitioner. I too, graduated with a liberal arts degree, decided to go in to an accelerated nursing program and went straight to NP school (with no RN work experience). The NP program will be considerable shorter and less money than MD. However, you must decide what you want from a future career. AS others have mentioned, do you want prestige, high pay, flexible hours, etc. It's true, there isn't much prestige for NPs and MDs make considerably more $. But, NPs have advanced education, and perform similar duties to MDs. Nursing school is very stressful and it is NOT easy! By 2015 all schools are going to change from entry level MSN to DNP. Those with an MSN before the transition will be grandfathered in. The DNP will be 3-4 years in addition to the BSN.
Personally, If I were really young and had a good shot at med school (2 years paid for), I would go that route. The career of ARNP is a great one, but there is still a stigma against us from some patients. Some patients consider us "junior doctors, wannabe doctors, etc. Although, that is not the case. We have a similar role to MDs, but we have a focus on the whole person and on prevention.
Good luck with your decision.