that's the question on most investor's mind i guess

and paying off the mortgage is every Australian's dream.
But on a Personal note ( I'm not a tax accountant or anything...just a simple mortgage broker

) - if your property is massively positive geared then you might consider buying other property to offset for tax reasons. As buying gold is NOT taxable...till you sell it ( and from my understanding it must be over a set value as well, i think over $1000- check with ATO)
So in summary,
Buying off Investment property- Since it's a investment property once it's pay off- you wont have the tax advantage any more.
Gold- Safe bet. hardly goes down in price. any amount to invest. only taxed at time of selling ( so if you wait till you retire!!! bonus! )
you could consider a bit of both , keep your money in both basket.