Once there is enough engineering-required work in front of you to act on, the first instinct is to hire an engineer. That makes sense on its face. Bring it in house, set the priorities yourself, keep the client relationship whole. For a contractor with steady, predictable work in a narrow set of areas the instinct is right. For most it solves less than it looks like it will.
Four places the two options diverge
Breadth of coverage
What it is. The range of specialties available to you on any given project. Arc flash modeling, protective device coordination, power quality investigation, relay settings, and commissioning support.
The trade-off. One engineer, however good, will be deep in some of these and thin in others. Building coordination curves is a different muscle from chasing down harmonic distortion. When a job lands outside their strongest area you are outsourcing anyway, on top of the salary you are already paying.
What it costs, and when
What it is. Whether the expense is fixed or moves with the work you actually win.
The trade-off. A hire is fixed overhead whether the work turns up that month or not. A partnership moves with your workload and gets marked up or passed through in the bid like any other subcontract. For most contractors that works out to more per hour and less per year.
Where the knowledge lives
What it is. Who accumulates the history on your clients, your project types, and how you like to work.
The trade-off. An in-house engineer learns your business and takes it with them when they go. A firm builds that same history across several people, so it survives somebody leaving. The catch is that you have to settle on one primary partner instead of shopping every job, or nothing accumulates at all.
Availability and control
What it is. Whether you can direct priorities when a client is waiting on you.
The trade-off. This is the real advantage of hiring and we would rather say so than pretend otherwise. Your engineer works on your schedule. A firm has other clients, so responsiveness has to be agreed at the start and then tested against your actual deadlines. It is a fair concern and a manageable one, and it is the thing to press hardest on while you are deciding.
Which one fits your firm
The comparison that actually helps is one person's knowledge against a whole firm's.
If your engineering work is steady, predictable and concentrated in one or two specialties, a hire will carry it and the fixed cost earns out. If it is varied, occasional, or growing in ways you can't forecast, a partnership covers more ground for less exposure.
Contractors who grow into more complex work rarely hire their way there. They get there alongside a firm that already carries the depth they would otherwise be trying to fit into one seat.