Simplicity favors templates. A single prompt is easy to build, test, version, and reason about — one input, one call, one output. A chain has more moving parts: multiple prompts, the plumbing that passes data between them, and more places for something to go wrong. If a task genuinely fits in one step, a chain just adds complexity.
Control and quality favor chaining for complex tasks. Decomposition lets you inspect intermediate results, fix a bad step before it contaminates the rest, and tune each step independently. With a single prompt for a complex task, you only see the final output — if it's wrong, you can't tell which part of the reasoning failed, and you re-run the whole thing.
Cost and latency cut both ways. A chain makes multiple model calls, so it can cost more in tokens and take longer end to end than one call — but each step can run on the cheapest model that clears its bar (extraction on a nano tier, reasoning on a frontier tier), which can make a well-designed chain cheaper than one big frontier-model call. The failure modes differ too: templates fail by being too rigid for an edge case; chains fail when an early error cascades down the sequence.