http://www.fa-mag.com/news/largest-etf-takes-in-most-cash-in-more-than-two-years-31640.html Investors flooded the world's largest exchange-traded fund on March 1 with the most new cash in more than two years, a spokesman said on Thursday, as they raced to capture gains in a soaring market. The SPDR S&P 500 ETF (SPY) attracted $8.2 billion on Wednesday, the largest inflows since December 2014 and the ninth largest figure of all time for the fund, according to the spokesman.
Why if one has cash available ? Borrowing costs are imbedded in ES, I'd go for ES as well rather than Spy if buying on margin, but not with cash. Talking about several days hold btw, I would prefer SPY either way for a day trade because of lower tick size.
I like the lower bid/offer spread on SPY, so yes SPY is better for day trading, but for buy and hold strategies for overnight or longer, ES (on margin of course) means cash is freed up to use elsewhere. Plus daily settlement of the futures means cash available in the account to be used elsewhere (assuming profit of course).