SPY and SPX both give you 0DTE exposure to the S&P 500, and both have daily expirations. But they are not interchangeable, and for a frequent 0DTE trader the differences add up fast.

Size and flexibility

SPX is about 10x the size of SPY (it tracks the index directly; SPY is ~1/10th). One SPX contract is a big position; SPY's smaller size gives you finer control over sizing and is friendlier to smaller accounts — you can scale in single contracts (0DTE position sizing). For most retail 0DTE scalpers, SPY's granularity is the practical choice.

Settlement: cash vs shares

SPX is cash-settled and European-style — no early assignment, and at expiration you just settle in cash. SPY is American-style and settles into shares, so a deep-in-the-money SPY option carries assignment risk if held to expiry. For 0DTE this matters: SPX's cash settlement removes the pin/assignment headache entirely (pin risk).

SPX is the cleaner instrument; SPY is the more flexible one. Most retail scalpers pick flexibility.

The tax angle

SPX options are Section 1256 contracts, taxed 60% long-term / 40% short-term regardless of holding period — a meaningful edge for an active trader versus SPY's standard short-term treatment. That alone pushes some high-volume traders to SPX. (Not tax advice — confirm with a professional.) NoVo trades SPY by design, for the sizing granularity and liquidity. See what 0DTE trading is and the 0DTE SPY scalping guide.