Where here's why I'm arguing it isn't the same: ES is not YM just as ES is not NQ and ES is not TF. Regardless of which one has the closest notional value to ES the component weightings and even individual components are different enough among the major indices such that this emulated e-micro ES contract won't hold up during any significant market volatility and even during non-volatile times may expose the trader to hidden divergences.
Here's a chart of 50*ES-5*YM since 2008. There are plenty of times here where if one went short on this spread (e.g. YM-ES) based on the behavior of ES they would have gotten more than they bargained for. I'd go so far to say as trading it during 2008 would be just plain dangerous.
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Every red area here is an example of where being long or short ES-YM (based on wanting to be long or short ES, respectively) would have resulted in some significantly different gains/losses than expected:
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If you want to trade this spread for *what it is* (strength of DOW components vs S&P500 components), then trade it. However, if you simply don't have enough money to trade ES, don't trade it - trade SPY.