Comparison
Truncation is a budget rule; compression is a selection decision.
Truncation removes context by position. Context compression scores or routes the source and can keep non-contiguous material relevant to a request.
Truncation is simple and predictable
A fixed head or tail is easy to implement and can be the right answer for append-only logs or a known recent-window policy.
Selection helps when answers are distributed
A failure identifier, cause, and remediation may appear in different parts of a result. A structural selector can retain those spans while keeping the source bytes.
Measure equal kept ratios
Compare answer coverage at the same achieved ratio. A smaller output alone is not evidence that a compressor preserved more useful information.