I think residency programs go through cyclical cycles and something which is very competitive today may not be so competitative after 5-10 years and vice versa. I work under a Interventional radiologist ( so he did his residency in radiology and than fellowship in interventional radiology) and have heard from him many times how easy radiology residency programs were 10 years ago. Radiology is highly competitive today just because of advancement in the field of radiology due to new toys ( MRI, CT etc.) led to better pay for the radiologists. Surgery has been going through the same cycle recently.
I think very soon Congress is going to step in and put a cap on how much a person can sue gynecologists as we are facing a huge shortage of physicians in this field and if this trend continues it may led to disaster as some towns/ cities may not have a single gynecologists ( already some small towns don't have them). This may help ob/gyn residency program and it may again become a lucrative field some day.
So you should follow your heart and do what you like. But i do agree that your top choice may change over 4 years in medical school.
This is just my 2 cents as I am still not a medical student ( but will be in four months
).
Mandar