Its always recommended to do electives in the specialty you are interested in. For example if you want to be a Gastroenterologist, its better to do electives in Gastroenterolgy or any other field of medicine; doing an electives in opthalmology or radiology wont be of much significance. Moreover there are many other benefits of doing electives in your specailty of choice, you establish good contacts in ur specialty of choice which may be of use later when you apply for residency, you might impress a program director during ur elective rotation and then things might be a bit easy for you during your interview in that program.

Visit the following links; they might help you in making your decision:

* Medical Specialty Aptitude Test(a test of 130 MCQs regarding your specialty)
* US Physicians work hours per week
* Physcician Specialty Demand & Salary
* How to Choose Your Specialty
* How to Choose Your Specialty Image
* Tough Specialties for IMGs