Every engagement I run is fixed-price. People ask me why often enough that I want to write the answer down once. The short version: hourly billing rewards me for being slow, and fixed price rewards both of us for being specific. I'll choose specific every time.
Hourly is the default in this industry, and the default is comfortable for the agency, not the client. The agency cannot lose. If the project goes long, more hours are billed. If the project goes short, the client paid for the going rate. The incentive to ship fast lives entirely on the client's side of the table, and incentives matter more than intentions.
Fixed price flips that. If I'm slow, that's my problem. If I'm fast, that's also my problem — I priced the work, I own the outcome. The client knows the number before we start, and the number doesn't move unless the scope does. When the scope does move, we negotiate it explicitly, in writing, before any work changes.
The objection I hear most often: "but you can't know what it'll take." That's true if the scope is vague. It's not true if the scope is specific. The scoping call is where I find out whether we can be specific yet. If we can't, I won't quote — and that's a useful signal too.
Scope creep is the thing fixed-price gets blamed for. In practice it's the thing fixed-price prevents, because every change has a cost that lands on the table instead of being absorbed silently. "Can we add this?" becomes "yes, here's what it costs, and what comes out." Trade-offs are visible. That makes them easier to make.
There's a moral component too. I've watched senior designers stretch easy work to meet hour minimums. I've done it myself, earlier in my career, when I billed by the day. It's corrosive. The work suffers and so does the relationship. Fixed price removes the temptation entirely.
What I won't do: monthly retainers, design subscriptions, "unlimited revisions" packages. All of these are forms of vagueness sold as flexibility. They make sense for some agencies — they don't make sense for the way I work, which is one engagement, one outcome, one ship date.
If you're choosing between hourly and fixed-price, the question I'd ask is: which model rewards the behaviour you want? If you want speed and clarity, the answer is fixed-price. If you want exploratory open-ended work without a defined outcome, neither model is right — you probably want a hire, not an engagement.