How on earth is your $220k apartment returning $15k net a year? I must assume you mean gross rent, else by the 50% rule you would have to be grossing $600 a week / $30k a year (13.6% yield!) to net $15k after strata fees / council rates / water rates / maintenance etc. More likely you mean $15k gross (6.8% yield, still very good in Aus AFAIK), so $7.5k net. Alternatively, that apartment might be worth a lot more than $220k...
At the very least, consider dumping that cash into something with higher yield (Ubank gives what, 4%?). Say Vanguard's Index Diversified Bond Fund if you are risk-averse (6%), or a share-market ETF otherwise (8-12%). There are both Aus and US options - given you have a source of income in Aus already, you may want to mostly focus on US stock ETFs or mutual funds to act as a hedge against exchange rate changes.
I am unfamiliar with SFIs, but REITs are basically a trust that pools money and uses it to develop property for capital gains, hold property for capital gains, or hold property for rent (sometimes all three). As I understand it, REITs tend to seek capital gains over the long term (more than one year) due to the favourable taxation treatment for capital gains in Australia (50% discount off marginal rate on gains from investments you hold for more than a year). One significant disadvantage of REITs from your position is that it increases your exposure to the property market directly. If it stagnates or falls, you will be out of the money in both your apartment (which may need to cut rent to attract a tenant in such an environment) and the REIT, so investing in an REIT is effectively doubling down on your confidence that the property market in Australia will continue to grow as it has.
Buying an additional property in Melbourne is little different to investing in an REIT, but with the advantage of being able to leverage (and negatively gear) but the disadvantage of less liquidity and higher transaction costs (mmm, stamp duty!).
The taxation treatment of your income in Australia will be relevant when you move to the US. I suggest you invest in meeting with a tax planner who is specialised in dealing with Aus/US clients, or possibly even one based in the US who can advise you on the US perspective. The taxation treatment of various investment options may differ substantially, to the extent it may be the determining factor in your investment choice. That is out of my depth. That said, be sure to investigate what happens to your superannuation when you leave Australia (can you transfer it to an IRA, or a regular account?).
For simplicity's sake I would consider selling the apartment if you decide to live in the US permanently. My brother has had experience managing properties from overseas (first a Melbourne apartment while in the UK, then a London townhouse while in Aus) and it was unpleasant. He had to burn a lot of social capital in order to get friends/relatives to assist with various matters that needed someone to do something in person.
Update: I read up on Westpac's Self-Funding Instalments (SFIs). They are to margin loans what an ETF is to a mutual fund: a pre-packaged margin loan, basically. With fixed leverage (1:1, or 50%) and some interesting taxation implications (capitalisation of interest) which I do not fully understand. Avoid like the plague, unless you really, really know what you are doing no exceptions!
If you must play with fire leverage, redraw against your wife's apartment in order to take advantage of the lower interest rate than you could get through any leverage scheme that is secured against shares. Simpler, cheaper, but still could mean you end up in the dumps if the share market takes a tumble. Don't say I didn't warn you...
