My advice would be to follow all 4 major index cons and decide for yourself which one you want to trade. You may want to trade them all and they all affect each other anyway. Some basic characteristics to compare would be tick size, price and average range.
It strikes me that this thread can't quite make up its mind whether it's about futures or index etf's (exchange traded funds). Some definition and clarity is required. Unless I'm very much mistaken, they are two very separate and distinct instruments. SPY, DIA and QQQQ are index etf's for the S&P 500, Dow Jones and Nasdaq 100 respectively which, for all intents and purposes, all trade like regular stock. Of the three, SPY is leaps and bounds ahead of the other two IMO, not least because of its volatility; a typical daily range of around $2.00 and because of its liquidity; the spread is rarely more than $0.01 cent and the volume is way up there in the millions - daily. For the benefit of the tread's OP, just to clarify; the QQQQ is the etf for the Nasdaq 100 and, therefore, is not the future of same - which is the NQ. They are two distinct and separate instruments and, although correlated, they trade quite differently from one another.