Here is the whole difference, in the only terms that matter when you are staring at a spreadsheet.
An ideal customer profile describes a company, in attributes you can filter on: industry, size, country, structure, what they sell and to whom. It decides which rows stay.
A buyer persona describes a person inside that company: what they own, what they get measured on, what they already tried, what they are afraid of. It decides what the message says once the row has survived.
So the answer to which one you need is both, in that order, and the reason the order matters is that only one of them can remove anybody from your list. If you build the persona first, you get a document full of fictional motivations attached to a list nobody has filtered yet.
The test that separates them
Put any line of your targeting document next to this question: can I filter a database on this today?
"Manufacturing companies in Germany and Austria, 50 to 500 employees, selling to other businesses, with an in-house sales team." That is an ICP. Every clause is a filter.
"Cares about efficiency, wants to grow, frustrated with manual processes." That is not an ICP and it is barely a persona. It describes most humans in business, and it removes nobody from the spreadsheet.
Anything that fails the filter test belongs in the message, not in the list criteria. That is the rule, and it settles most arguments about which document you are actually writing.
Where the titles come from
This is where the two documents meet, and where most lists go wrong before a single email is written.
Go through your last ten or twenty closed deals and work out who genuinely made the call. Not who you pitched. Not who attended. The person who said yes, and the person who could have killed it alone. Then use those exact titles.
Most lists I see on the first day of an engagement have a title column somebody guessed at, usually filled with the word "Manager". In most B2B niches that word describes somebody who cannot approve anything, and it drags in operations, HR and whoever else carries the same badge. Generic titles like Assistant, Manager and Associate come out of the criteria before the list gets built.
Real titles are unglamorous and specific. Head of Production. Technical Director. Owner. Those come from your own history rather than from a template.
The persona has a job, and it starts later
Once the rows are filtered, the persona does real work. It is what stops you writing to a Technical Director about board-level growth strategy.
The cost of skipping it is not subtle. A message aimed at nobody in particular reads like one, and the senior people you most want to reach are the quickest to spot it, because their inbox is where everybody else's guesswork lands too.
That knowledge cannot come from an ICP. An ICP tells you the company has 200 employees in Bavaria. It does not tell you that the person you are writing to spends their week arguing about lead times.
One persona is already the wrong shape
The other reason not to start with a persona: there is rarely one of them.
One benchmark roundup, citing Gartner, reports that buying committees on deals over $100,000 now run five to eleven people, up from four to six in 2017. Smaller deals are lighter, but the direction is the same everywhere. A single invented buyer named "Operations Oliver" is a picture of one chair in a room that keeps adding chairs.
What you need instead is the account, filtered properly, and two or three real roles inside it with different concerns. That is a list you can work and a set of messages that can differ from each other.
The ICP comes first, and it comes from deals you already won
Attributes you can filter on, taken from contracts that exist. Decision maker titles pulled out of those same deals. Then persona detail for the two or three roles that actually turn up in the room, shaping the words rather than the rows.
If you are about to build the list itself, the procedure sits in building a B2B lead list that is worth sending to. Choosing which market all of this points at is a bigger decision upstream of both documents, and I worked through that one in how I pick the market.
One warning to leave you with. A persona written before you have closed anything is fiction, and an ICP written the same way is a guess with a spreadsheet attached. Both get much better after your first handful of signed contracts, which is roughly the moment most companies stop updating them. It is also the moment narrowing to one offer and one audience starts paying for itself.