Sector funds: SPDR vs Vanguard vs Fidelity

The sector ETFs from the different fund houses track different indices.

The Fidelity sector ETFs track the MSCI USA IMI sector indices. (IMI = Investable Market Index, covering 99% of the market capitalization of the country).

The Vanguard ETFs track the MSCI US Investable Market 25/50 sector indices. "25/50" indices are structured to be fully compliant with RIC regulations, capping a single issuer at 25% of the index, and capping the sum of the weights of all issuers representing more than 5% weight at 50% of the index.

The SPDR ETFs track Select Sector indices, which draw exclusively from the S&P 500.

Overall, the Fidelity and Vanguard sector ETFs are more diversified, and have more small-cap exposure compared to the SPDR sector ETFs, which are large-cap focused.