Private loans are Ok for medical school, but the federal loans are typically a better deal. The federal loans will typically have a lower rate and offer better terms. The Stafford loans require no payments while in medical school and the interest on the loan is only capitalized when you are finished with school. Some private loans will capitalize interest on an annual basis meaning the loan will earn interest on interest. Some private loans also require some interest payments while in school. The other problem with private loans is that they will more than likely want to see your credit history and can deny you the loan or require a co-signer. Granting of federal loans has no basis on credit history (so long as you have not delinquent on any other federal loans).
So although private loans are fine if that is all you can get, be sure to examine the loan specifics to be sure you are not getting ripped off.