If you pay the interest to avoid it capitalizing, you're leaving more principal to pay interest on, so the next effect is the same. For example (using made up numbers):

Loan 1: $10,000 principal + $1,000 interest

Loan 2: $20,000 principal + $2,000 interest

Pay off the $3,000 in interest, and you're left with $30,000 that will be charged interest. Pay off $1,000 in interest plus $2,000 in principal on Loan 1, and you're left with $30,000 that will be charged interest ($8,000 remaining principal on Loan 1, $20,000 principal on loan 2, and $2,000 capitalized interest on Loan 2).

So my recommendation would be to put it toward the highest interest loan, assuming you anticipate being able to make the payments when they start in November. Also, I'd adjust your withholdings so you don't get such a big refund next year :)