The main attraction of property investment over equity investment is the tax treatement of interest on the mortgage.
If you are not borrowing to invest, you should probably invest in equities - higher return, less risk and less hassle, although greater short term volatility.
If the attraction is borrowing, then you should borrow the maximum. You must be able to tolerate the following risks:
periods of no rental income
Declining rental income
Increased interest rates
It's likely that you will be tolerating these risks from your non-rental income.
Brendan