Model comparison is the structured practice of running multiple models against the same set of inputs and measuring how they differ. This might compare two versions of the same model (to evaluate a training update), models from different providers (to inform a build-vs-buy decision), or a finetuned version against a base model. Meaningful model comparison requires careful experimental design — the same prompts, the same evaluation criteria, and a large enough sample to distinguish real differences from random variation. For behavior architects, model comparison is central to strategic decisions about which model to deploy: it grounds those decisions in behavioral evidence rather than provider reputation or benchmark scores that may not reflect your specific use case.