When evaluating S&P 500 exchange-traded funds (ETFs), investors frequently prioritize expense ratios, leading many to believe that the State Street SPDR Portfolio S&P 500 ETF (SPYM) offers a superior value proposition compared to the Vanguard S&P 500 ETF (VOO). While SPYM's reported gross expense ratio is indeed marginally lower than VOO's, a deeper examination of their underlying methodologies reveals nuances in their long-term performance records that demand closer scrutiny. A crucial footnote in SPYM's performance table indicates a historical reliance on multiple indexes, a detail that significantly impacts direct comparisons.
The primary attraction of SPYM for many investors lies in its expense ratio, which stands at 0.02%, a single basis point less than VOO's 0.03%. This minute difference in fees is often the deciding factor for investors seeking the most cost-effective way to gain exposure to the S&P 500 index. State Street Global Advisors, the fund's manager, highlights SPYM's low trading costs, with a 30-day median bid-ask spread of 0.01% as of October 1, 2026. On the same date, SPYM held 506 securities and managed assets totaling $173.90 billion, underscoring its significant market presence.
However, the key distinction emerges when analyzing SPYM's benchmark history. State Street Global Advisors openly states that SPYM's benchmark reflects a linked performance derived from four distinct indexes over its lifetime: the Dow Jones U.S. Large-Cap Total Stock Market Index from inception until July 9, 2013; the Russell 1000 Index from July 9, 2013, to November 16, 2017; the SSGA Large Cap Index (an internal State Street index) from November 16, 2017, to January 24, 2020; and finally, the S&P 500 Index from January 24, 2020, to the present. This means that SPYM's stated "since-inception" performance, and even its ten-year average annual NAV total return of 15.36% (as of August 31, 2026), are a composite of these varying benchmarks. In contrast, VOO has consistently tracked the S&P 500 throughout its history, ensuring a direct and uninterrupted comparison to that specific index.
Despite these historical benchmark differences, the recent performance figures show remarkable convergence. From January 24, 2020, to October 2, 2026, a period during which both funds tracked the S&P 500, SPYM's adjusted price increased by 158.75%, closely mirroring VOO's 158.39%. Over a broader ten-year span ending on the same date, SPYM's return was 324.49%, while VOO's was 322.34%. This close alignment in returns suggests that while SPYM's historical benchmarking strategy introduces complexity, its recent performance, when aligned with the S&P 500, has been nearly identical to VOO.
Both funds exhibit a significant concentration in information technology, with SPYM reporting 39.94% of its holdings in this sector as of October 1, 2026. NVIDIA represented the largest individual holding, accounting for 8.44%. This sector weighting is a direct reflection of the S&P 500 index structure, implying that neither fund alone provides significant portfolio diversification beyond large-cap U.S. equities. For investors seeking broader balance, supplementary investments would be necessary. While SPYM currently offers a slight cost advantage and matches VOO's S&P 500 tracking, the historical shifts in its benchmark index are a critical factor that investors should consider when making long-term comparisons.
