The choice follows from one question: how well can you describe in advance what has to happen? The sharper that answer, the more fixed the remuneration form can be.
Fixed monthly fee
Choose this when the work returns every month and you want a team that knows your brand. You are buying availability and continuity, with a predictable cost.
Then write down exactly what it contains, expressed in work rather than hours: how many campaigns, how many adaptations, how many meetings. Without that list the scope grows a little every month, and someone has a painful conversation a year later.
Project price
Choose this when the assignment has a beginning and an end and you can describe the scope. The agency carries the risk of surprises, you get an amount.
Then write down how many feedback rounds are included and what happens on a change. Nine out of ten overruns come from that, not from a wrong estimate.
Hourly rate
Choose this only for work you cannot bound: research, coaching, a crisis, an exploratory phase. It is the fairest form for uncertainty and the most expensive for routine.
Then write down a ceiling per phase and a mandatory alert when that ceiling comes into view. An hourly rate without a ceiling is an open account.
Commission on media spend
Choose this carefully. The model rewards volume: the more you spend, the more your agency earns, including when spending less would be better. If you do work this way, agree that advice on the budget level is separate from its execution, or have that advice given by another party.
The combination that works most often
A fixed fee for the running work, project prices for anything on top, and an hourly rate as a fallback for whatever fits neither. Three forms in one contract, each with its limit stated, is clearer than one form stretched over everything.
Which model suits which assignment, and what you settle alongside it, is the conversation IKAg runs under Agency evaluation & remuneration.
See also: which remuneration models exist and how to keep costs from running away.