mmm - interesting/depressing that this topic was going on a year ago and i have some arrogont learners on another thread now, who dont know diddly, telling me i am wrong about what is a very simple and basic part of trading
to trade the S&P 500, you have to use futures or etfs or a parcel of stocks of all the constituents of the S&P500, or a parcel of a smaller number of stocks that reasonably replicate the S&P500
but the reason you are doing it is to make money and futures give high leverage and transparancy - and because of that, the best traders are going to be using them to make a lot of bucks each day
as the traders are trading the futures contract - the contract will move up and down or sideways with their sentiment and needs - but the consensus of these traders do the exact right thing at any given moment - even if that changes every moment - so effectivly, the futures are always going to lead the cash index, since these traders are always one step ahead, so if you trade off the cash index - and not the futures - you are always gonna be that bit behind
there are an awful lot more technical and trading reasons why you would definetly not want to use the cash index and a lot more reasons why you would want to use futures to guide you - but regardless of them - the simple fact outlined above - if you think about it - should be enough to get you straight
if it isnt enough- dont trade until it is - or you are going to be dumping money left right and centre
dont worry about what is happening with the cash index - your bet is riding on the futures - so screw the cash index - forget all about it