Work in progress…

The Russell 3000 is generally better for tracking market breadth compared to the S&P 1500 for several reasons:

1. The Russell 3000 provides broader market coverage, representing approximately 98% of the investable U.S. equity market compared to the S&P 1500's 90%.
2. The Russell 3000 includes a greater number of smaller companies, which makes it more comprehensive for measuring the breadth of market participation across the full spectrum of U.S. equities.
3. The Russell 3000 uses a purely rules-based, objective methodology for stock selection, whereas the S&P 1500 uses a committee-based approach with subjective elements.

However, the S&P 1500 has some advantages for certain investors:

1. The S&P 1500 incorporates an earnings screen that filters out lower-quality companies, which may provide a more accurate representation of the tradeable market universe.
2. The S&P 1500 typically has lower turnover due to its as-needed approach to constituent changes versus Russell's annual reconstitution, potentially making it more stable for tracking purposes.
3. The S&P 1500 tends to avoid less liquid, lower-priced stocks, which could provide a more realistic view of the investable market breadth.

For monitoring market breadth specifically, some analysts use the S&P 1500 to track the number of stocks within 5% of their 52-week highs as an indicator of market health and breadth.

https://www.wstam.com/news/market-updates/june-2021-global-equity-markets-review/

Your choice between these indices should depend on whether you prioritize comprehensive market coverage (Russell 3000) or a more quality-focused representation of the tradeable market (S&P 1500).