If you have enough money to pay extra on your mortgage, save it! Paying extra on your mortgage doesn't help much. If by chance you ever fall on hard times (lose your job) and can't make payments, they won't care that you made extra payments for x years, they will still foreclose and you will be out out in the cold. Instead, if you save the extra, you can at least cover your mortgage with the savings until things get better.

Also, why worry so much about paying off the lowest interest loan you are likely to ever get?