So much depends on the product and the broker. At one time the cutoff for the SPX for TD/TOS was under 10 contracts went to the COB, while over went to the pit and the broker had the option of putting in the COB. Not sure if that has changed. I have had different experiences at IB, they will not tell me what their cut off is in the SPX. Recently put a 15 lot spread order in and it went to the COB. The SPX and the SPXW (weeklies) are treated differently as well. Generally all the SPXW orders will be sent to the COB, as opposed the regular SPX. As Robert Morse mentioned, customer orders and professional orders are treated differently as well. All professional orders in the SPX, both spreads and single orders, are routed to brokers (unlike SPXW orders) This makes the SPX almost untradeable as a "smaller" professional, and this is why I gave up my prop account. When you get to multiple listed equities and indexes it is much harder to tell where they go. Exchanges have different rules for payment for order flow for their cob's. For example the CBOE does not charge/pay for an order sent to their cob whether taking or providing liquidity. So a broker may be incentivized to send your spread to another exchange where it will be paid, and you may not receive the best fill there. I generally receive the best fills in equities on the CBOE cob.
If you have a Portfolio Margin account without excessive add on margin, and only trade during "regular" trading hours, SPX is better than ES. Otherwise, I agree. In general, I'd rather focus on ES.