I've essentially done the same thing as a margin loan, albeit for a smaller cash loan ($75k vs $100k). With the margin loan, there is the daily interest cost. With the futures "loan", the interest is baked in because the futures quote for a later date will be higher than the current index quote.
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How can we compare the interest rates? For the margin loan it is simple and is what the brokerage gives us, say 6%. The cost of 1 year borrowing $100k is $6k.
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For the futures trade, let's look at an example with current quotes. Say the current S&P index is 6466. A futures contract of length 117 days is 6541. So the upfront cost is `6541-6466 = 75`. That is `75/117` per day or `((75/117)*365)` per year: $234.
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That means for our loan of $75k, the interest rate is `(234/75000)` or about 0.3%. That is much lower than 6%.
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This example is mainly to show that these loan instruments are interchangeable and it all comes down to how you achieve leverage.