COMPARISON
One is a vendor relationship. The other is an organizational capability. The difference determines whether marketing is an expense or an engine.
Neither model is universally better. Agencies provide skills and speed that internal teams cannot match. Systems provide integration and accountability that agencies cannot achieve. The best approach depends on what the business needs, what it can afford, and what stage it is in.
The mistake is treating the choice as binary. Most businesses need both: agency partnerships for specific capabilities, and internal systems for ongoing execution. The question is not which to choose. It is whether you know what you need and are honest about what you are actually getting.