At the end of the day - it all depends on whether you can cover the mortgage (plus operating expenses plus any capital expenses) then fine. It seems like you got your answer but cats were calculating the two mortgages as if debt service and rent wer the only consideration.
If you are upside down considering capital expenses, etc,, you might have to refi out, making any necessary improvements, and put some money in a reserve fund. Paying down the mortgage is all good if you first have enough money in ya reserve fund for preventive maintenance and in case shit happens funds. 15/30 doesn't mean shit when the tenants fuck up the property and the security deposit won't cover (and they have no money - so good luck on taking that to court).
The question you should ask is: where is the rental market going? Can you cover right now? Are there any improvements that need to be made?