[creationix]

You're right. Some important differences:

sick is binary, rx is textual (this matters for tooling)

sick has size limits (65534 max keys for example. I have real-world rx datasets reaching this size already) rx uses arbitrary precision variable-length b64 integers. There are no size limits anywhere inherit in the format, just in implementations.

sick does not preserve object key order rx preserves object key order, but still implements O(log2 N) lookups for object keys.

etc.