If you have any mortgage on a rental property, you can deduct the interest paid on the mortgage against the rent received in calculating your taxable profit. If you have a mortgage on your home, you can only claim tax relief at the 20% rate up to a certain limit. So, if you have two properties, you want as high a mortgage as possible against the rental property and as low as possible against your home.
So your broker is correct, if you are planning to let the property in three years time. However, the amount of capital repaid in the first three years would be very small, so it won't make that much of a difference.
Brendan