When does it make sense to pre-pay mortgage?

[Sean Golin]

I currently own an investment property and have been considering pre-paying the mortgage down to increase equity but not sure if this is the right approach.  My mind tends to focus on the increased monthly cash flow from the property assuming a paid-off mortgage - but I know I could also consider buying additional properties.  

Is there an alternative that is better than the other?  Any info would be appreciated.  I've included some details below:

Purchase Price:  $280K

Down Payment:  25%

Interest Rate:  4.75% (30-yr fixed)

Net Monthly Cash Flow:  Average of +$500 per month (after mortgage, taxes, ins, repair, vacancy)

Est. Current Property Value:  ~$310K

Thanks!