If you're buying a bike for someone other than yourself, make certain the recipient can swap if for another size. That isn't always the case with closeouts or demo models.

When you're down to 2 frames of similar size, it can come down to riding style. Do you like an aggressive position or more relaxed? A 56 will have more saddle to bar drop for most riders. A 58 might have less.

Also consider that a 56 will typically have a shorter headtube, shorter cranks, narrower bars, shorter stem and possibly a different fork angle.