I will vote futures as having the greater influence. This is my theory. Index futures are not subjected to demand and supply like the way the underlying stocks are. To sell stocks, you have to own stocks. To sell futures, you don't need ownership as long as you are willing to open a contract to sell. During panicky times, I would say it is futures who drive stock prices down more than the other way round. This is what happened in the 2010 flash crash and the great intra-day crash in 1987 started in the index futures market. Just my theory and not validated.