Direction
SSE = server → client only. WebSocket = full duplex.
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Simplicity
SSE is HTTP. Any HTTP server can produce it. Firewalls, proxies, CDNs pass it. WebSocket is a separate protocol with LB, upgrade, and framing considerations.
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Message rate
SSE + HTTP overhead per event is small but real. Under 100 events/sec fine. Over that, WebSocket's binary framing wins.