Generated September 19, 2026.
Overview
FXAIX is a broad S&P 500 index mutual fund that tracks the market-cap-weighted 500 largest U.S. stocks. SPMO is an ETF that narrows the S&P 500 universe to the highest-momentum names within the index. The core distinction: FXAIX delivers the full market return at a near-zero cost, while SPMO tilts toward recent price strength and charges for that active factor bet.
How they differ
FXAIX holds the entire S&P 500 in market-weight proportion, so it tracks the index directly with minimal deviation. SPMO filters the S&P 500 for momentum—stocks with the strongest recent relative performance—meaning its holdings diverge from the broad market and its returns will differ materially depending on whether momentum is outperforming or underperforming. Third, SPMO carries a 1.35 beta compared to FXAIX's 1.0, indicating that SPMO amplifies market swings—it's a more volatile play on U.S. equities, not a steadier holding.
Who each is best for
FXAIX: Fits investors seeking straightforward, low-friction exposure to the broad U.S. large-cap market, with minimal ongoing fees eating into returns. Works well for long-term accumulators who prefer simplicity and don't expect to beat the market, only to match it.
SPMO: Designed for investors with a conviction that momentum—stocks with strong recent price trends—will outperform the market over their time horizon, and who are comfortable with higher volatility to chase that edge. Suits traders and tactical allocators willing to accept factor drawdowns in exchange for potential factor outperformance.
Key risks to know
- Factor timing and reversion risk: Momentum is cyclical. SPMO can significantly lag broad-market returns during periods when slower-moving or value-oriented stocks lead. The factor outperformance that justifies its higher expense ratio is not guaranteed and may take years to materialize, if at all.
- Concentration within the factor: By screening the S&P 500 for momentum, SPMO will hold fewer names and with heavier weights in the strongest performers. This creates concentration risk within the momentum subset—if the most-momentum names stumble together, the drawdown can exceed the broader market.
- Beta amplification in downturns: SPMO's 1.35 beta means it will fall roughly 35% faster than the S&P 500 in a market decline. Investors expecting a quick rebound may see steeper short-term losses. Over a decade, that's a meaningful hurdle.
Bottom line
If you want broad, low-cost market participation and don't expect to beat the index, FXAIX's 0.015% and $859B in assets make it a straightforward choice. If you believe momentum will outpace the market and accept higher volatility to pursue that premium, SPMO's factor tilt may appeal—but recognize that factor bets underperform regularly and its costs are measurably higher. Past performance of momentum or broad-market strategies does not predict future results.
AI-generated analysis for educational purposes only. Verify important details independently; past performance does not guarantee future results.