Let's compare what $10K will get you today using all three approaches.
Assuming you are using standard RegT 2:1 overnight margin requirements, your $10K becomes $20K, and that's how much you control. Currently that's 112 shares of SPY.
How much leverage you use depends on the moneyness of the option.
Let's look at the Feb Week2 (expiring Feb 14th 2014) options. With the nearest ITM SPY call (177), you can achieve a large leverage. With $10,000, you can buy 50, at roughly $1.97 each, controlling 5000 shares of SPY, roughly totaling around $900,000, or 90X leverage. However, the disadvantage is that the delta of these options is only about 0.55, as the time premium is a whopping $1.47. So if the stock doesn't move much, you are out a huge portion of your investment (roughly 75% of it, which was time premium). So by the stock doing nothing, you lose a lot. And if the stock goes up, it will only be matched 55 cents on the dollar, so you are really controlling more like $500,000, not $900,000.
Compare this with the 171 call. The price is $6.81 and the spread is large, but volume is sufficient to buy a few calls. With your $10K you could buy 14 calls, controlling 1400 shares of SPY, totaling about $250,000, or 25X leverage. So, while the leverage is much less, the risk is also substantially reduced. If the stock goes nowhere, you lose only $0.14 of time premium, or about 3% of your initial investment, as the intrinsic value of the option is so high.
The notional value of 1 emini contract: $1767 (current price) x $50 per dollar = $88,350. So, with your $10K, you can control about $176,700, or about 17X leverage, since the initial margin is $4,758 per contract.
So there you have it. The greatest leverage is with the ATM option, the next is with the deep ITM option, next the emini contract, and finally the outright stock. Probably the best bets to leverage your money are the deep ITM calls and the emini futures. This is not a recommendation in any way, just a presentation of the data. I did this rather quickly so hopefully I didn't make any errors in my calculations; if I did please let me know!
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