The biggest difference between Alpaca Data API and Polygon is the density of the data.

While Polygon data is full volume consolidated from all exchanges in the U.S., Alpaca Data API consists of five exchanges (IEX (Investors Exchange LLC, NYSE National, Inc. Nasdaq BX, Inc, Nasdaq PSX, NYSE Chicago, Inc.). That said, Alpaca Data API satisfies most of the daily uses with enough accuracy as to the real time price needs.

Due to the exchange requirements, it is necessary to have a live trading account and sign the agreement to start accessing Polygon data. Users without a live brokerage account have access to Alpaca Data API.

For those who have Polygon access via Alpaca API key, Alpaca Data API still works with the same key, and applies separate constraint for the concurrent connection. You can also use Alpaca Data API streaming for the purpose of backup and redundancy.

Alpaca Data API provides more flexible query capability such as multi-symbol and precise time range parameters, in addition to the real time trade & quotes data. Alpaca’s intention is always to extend the API such that different use cases are easily met. If you find different needs for API design, please send us any feedback, too.

For more information read here.

Market Data - Documentation | Alpaca
Alpaca API lets you build and trade with real-time market data for free.