Another point to remember is that you only get a tax benefit for mortgage interest on your primary residence if your itemized deductions exceed the standard deduction. So, it may be more tax-beneficial to pay down your primary residence first. There are a lot of people out there who don't benefit at all from mortgage interest deductions on their primary residence, particularly if they don't have an expensive home, and don't have many other deductions.
Interest paid for a mortgage on a rental property is always tax deductible since it counts against income (unless your rental income is negative once you factor in depreciation, which gets more complicated).
Also, I echo icefr's comment about PMI. If you can pay down the balance to get rid of PMI, then do that. Otherwise, if property values are increasing in your area, you could also look into the bank's rules on getting it reappraised and seeing if you could get rid of PMI, even without paying down your mortgage further at some point.