I see no reason not to get your First Class Medical initially if you don't mind coming back 6 months after. However, the most important thing is to take care of your health to ensure you can keep passing and renewing those First Class Medical Exams.
is a chart from an aviation medical-related website that compares the difference between a: First, Second, and Third Class Medical Exam.
Click Here for Link
According to that link, the First and Second Medical Exams are identical for the exception of the "ELECTRO-CARDIOGRAM" that is required at age 35 and annually after age 40 for the First Class Medical. So if I were you, since you are working on ratings for now, I would opt for the Second Class Medical since it is almost the same (unless you are 35 years old or 40+) and save some Time and Money.
In addition, check on the flight school you plan on attending because some (usually flight academies and universities)require you to possess a First or Second Medical.
Also, I would ask your Aviation Medical Examiner during your exam if he thinks you will be able to pass a First Class Medical.
EDIT: But it ALL depends on what you want... if you feel more comfortable with a First Class Medical because you will know you can pass the exam, then go for it.