I agree with MDM- look at the tax benefits from each:
The student loan interest is a tax deduction whether or not you itemize. So, your "true" interest rate is going to be less that 7%, depending on your tax bracket.
Most rental properties provide tax benefits (because you can claim depreciation, etc.), so you are likely getting tax breaks there, too, which would make your "effective" returns higher than 15%. Rents should go up ~2%/yr, while your mortgage stays the same.
Without looking at actual numbers, I'm guessing it makes most sense to keep the rental and pay down the loan aggressively (like you are).