I think spending some time with nurses and doctors will help you decide which path you will want to take. Both fields, while similar, are also very different in terms of tasks, educational requirement, hours, pay, etc. If you have time, see if you can spend some time with other healthcare professions, like NP or PA, Respiratory therapist, Physical therapist, etc. They all have their plus and minuses. Get a feel of what their average day is like and try to see if you can picture yourself in that role on a daily basis.
If you're almost done with your pre-reqs for Med School, there is no need to wait till 2009 to take the MCAT. As long as you are done with Biology, General Chemistry, Organic Chemistry, and Physics, you can take the MCAT when you feel ready. These are the subjects tested on so it would be wise to have these courses material mastered before you decide to take it. Most pre-meds spend 4-6 months studying for it ... some longer, some shorter. There is also a verbal component to it that is similar to the verbal component of the SAT.
Student Loans - when you enroll in an accredited school (either nursing or medical school), if your loan was from a reputable bank, you may be able to place them into deferment while in school ... meaning you don't have to pay them while in school. If any part of your loan is federally subsidized (stafford loans, perkins), the federal government will pay off the interests while in deferment. Call your bank and see what your options are should you go back to school.
Follow the path that you think will be best for you. If you have some sense of fiscal responsibilities, the loans, the debt shouldn't be a burden as long as you keep focus on the big picture and long term goal ... not just income but also lifestyle and your happiness.
It's normal to worry about debt - it a good sign of responsibility and maturity. This might help should you decide to pursue med school instead of nursing school
Here is a 2004 survery of average compensation for various medical specialities
Anesthesiology - $332,216
Emergency Medicine - $246,760
Family Practice - $148,563
General Surgery - $256,111
Internal Medicine - $158,500
OB/GYN - $221,286
Pediatrics - $131,000
Psychiatry - $182,300
Source: Jackson and Harris Physician Compensation Survey, May 2004 (base compensation, incentives and/or production)
EDIT: (adding this info)
According to salary.com, the national average salary for "staff nurse - RN" is $59,046 - however, you can easily make much more than that depending on your workhours (overtime, which shift you work), what part of your country you're in, and what field you work in
In the journal "Nursing" Oct 2004 issue by Robinson, Eileen. and Mee, Cheryl, average annual income was $54,574, with more than 25% of those surveyed making more than $65,000. Average income for ICU/CCU nurse was $58,400, Nurse Supervisor/Manager was $67,100.
LPN/LVN average income for 2004 was $32,400.
For NPs, the average income for 2004 was $73, 235 with a standard deviation of $20,505. 5th percentile was $38,000 and 95th percentile was $220,000
For CRNA, according to salary.com, average income was $130,776
Average income for physical therapy (PT) ranges from $55k-$75k
Source: Journal of Physical Therapy Education, Fall 2004. Article by Redman-Bentley, Donna