Here is a link for a general comparison of the gradplus vs private loans:http://www.wm.edu/financialaid/gradplusloan.pdf
Also, each financial instutuion offering the loan are throwing in their own incentives. We have been spoiled these past few years by low interest rates, especially on student loans but with private loans it is not unconceivable for the effective rate to reach upwards of 12 to 14 percent. You can research historic prime and libor rates and this might help with your decision to verify this but historic rates are not an indicator of future rates.