Emini futures are very popular with traders these days, and its true you'll get better tax rates and leverage, but the downside of trading the Emini's, when you look at your total costs, trading the SPY is alot cheaper.
Heres the breakdown:
Every 4 ticks (or 1 point) on the ES = 10 Cents on the SPY.
So 1 tick on the ES = 2 ½ cents on the SPY.
This means every time you make a trade with the ES, the market has to move an extra 1 ½ cents to cover the spread (before you can even think about making a profit on the trade).
Then theres commissions, trading the SPY, its like a stock, you can have a fixed commission with unlimited shares per trade. With futures you have to pay a seprate commisson for each contract (usually about $4 roundtrip).
Heres an example of a trade with the SPY:
You buy 5000 Shares at $110.50, the price has to move to $110.71 for you to make 2 points ($1000) on your trade.
Your commission is $14 roundtrip ($7 + $7) so your total profit is $986.
Heres an example of the same trade with the ES:
You buy 10 contracts at 1100.50, the price has to move (9 ticks) to 1102.75 (thats $110.50 to $110.73 on the SPY) for you to make 2 points ($1000) on your trade.
Your commission is $40 roundtrip (10 X $4) so your total profit is $960.