A project manager at a commercial GC put it plainly: on public work it is typically the low bid. On private work it comes down to time of completion and cost.
That difference is the whole thing, and most contractors market as if only the first one exists.
Public bids are a different game
On a public job, the process is designed to remove judgment from the decision. Sealed bids, published requirements, lowest responsible bidder wins.
There is very little a contractor can do to be chosen for reasons other than price, because the process exists specifically to prevent that.
Marketing does almost nothing here. That is not a failure of marketing, it is the point of the system.
Private bids are decided earlier than the bid
On private work, something different happens. The client is weighing cost against schedule, risk, and whether they believe you will actually deliver what you said.
Which means by the time the numbers come in, the client usually has a preference. The bid confirms a decision more often than it creates one.
That preference gets formed somewhere. Usually a referral, a past project, a relationship with an architect or a developer, or something they saw and remembered.
What a selection committee actually looks like
On institutional and healthcare work, you are frequently not being chosen by one person.
There is a facilities director who cares about schedule and disruption. A finance person who cares about the number. Sometimes a board member or an administrator who has never built anything and is voting on gut feel.
The person who has never built anything still gets a vote. If they cannot follow what makes you different, they default to the number, because the number is the one thing they understand.
The gap most contractors have
Ask a GC what makes them different and you tend to get the same short list. Relationship driven. High percentage of repeat clients. Communication. Problem solving. Safety.
Those are usually true. They are also what every firm bidding the same job is saying, which means they cannot be used to choose between you.
The interesting version is one layer down. Eighty percent repeat business is not an opinion, it is a number, and the clients who keep hiring you can say why on camera. That is a different kind of claim than saying you are relationship driven.
What actually travels
Three things tend to move a private selection.
Somebody who worked with you saying what changed. Not that they enjoyed it. What was different because you were the one doing it.
Proof you handled something going wrong. Every project has a hard moment. How you handled it is more persuasive than a project where nothing happened.
Something the non builder on the committee can follow. If your differentiator only makes sense to another contractor, it does not survive the room.
Short version
Public bids go to the low number. There is not much marketing can do about that.
Private bids are usually decided before the numbers arrive, based on preference formed earlier.
Committees often include people who have never built anything, and they vote on what they can follow.
Relationship driven, communication, and safety are said by every firm bidding the same job, so they cannot break a tie.
What breaks a tie is a specific claim only you can make, said by somebody other than you.
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