The choice is not about money but about who ties the work together. With one agency, they do; with several, you do, or nobody does.
What one agency gives you
One briefing, one plan, one contact and one party responsible when things go wrong. That last point is worth more than it sounds: with several parties everyone points at the other when there is a problem, and it costs you the time to work out who is right.
What you give up: you buy their weakest discipline along with the rest. Every agency is good at something and mediocre at something else, and in a full-service contract that mediocrity is included.
What several agencies give you
For each craft the party that is best at it, and a price that stays sharper because nobody holds a monopoly on your budget. For large advertisers this is the standard form: a creative agency, a media agency, and one or two specialists.
What it costs: coordination. Count half a day a week of alignment per extra party, and someone on your side who can and may do it. Without that person three agencies head in three directions and you pay three times for the same research.
The rule of thumb
- One agency if your marketing team is small, your budget limited, or you mainly need coherence.
- Several if you have a strong internal orchestrator, if the crafts genuinely diverge, or if your budget is large enough for specialisation to pay off.
- Never several without a clear demarcation of who decides what. Two agencies that are both "the brand guardian" is a guaranteed fight.
The practical compromise
One lead agency that guards the line, with specialists around it that report to it or at least align with it. Put in both contracts that working with the other party is part of the assignment; otherwise it is a favour rather than an agreement.
Lining up several parties and dividing the roles is exactly what IKAg calls an agency ecosystem: see Agency ecosystem.
See also: the difference between a creative and a media agency and what you do yourself and what you outsource.