Legality in California
Both are CFTC-regulated event-contract platforms, legal in all 50 US states including California. No meaningful regulatory difference between them for California residents.
Markets and coverage
Kalshi covers sports, politics, economics, culture, crypto, and weather: the broadest coverage in the US market. Sleeper focuses on sports (NFL, NBA, MLB, NHL, soccer) with growing player prop depth and selected cultural markets. If you only trade sports, both work. If you want politics or Fed markets, only Kalshi delivers.
Pricing & fees
Both charge modest per-contract fees disclosed at order entry. Kalshi's fees are typically competitive with traditional futures exchanges. Sleeper's are in a similar range. Neither has a built-in house margin like a sportsbook.
User experience
Kalshi's interface is finance-forward: markets organized by category, clean order flow, feels like a trading app. Sleeper's interface is social-forward. Your fantasy leagues and group chats live in the same app as your trades. The right answer depends on whether you want focus or community.
Who should use which
Use Kalshi if: you want broad market coverage, you're not already on Sleeper for fantasy, or you want a more focused trading experience. Use Sleeper if: your fantasy leagues live on Sleeper already, you primarily trade NFL and NBA game-by-game, or you want trading and fantasy in one app.