(caveat: I haven't worked with either but they are highly regarded)
Pro tip:
You can usually gauge an agency's sophistication by their content marketing.
If their blog/twitter/CEO etc. regurgitate grade school level tactics...run.
In the absence of referrals, the best tactical content usually leads to competent practitioners.
Agency Pro #1: domain expertise in marketing platforms
Agencies typically run a ton of ad spend through their channels of choice
Because of this, they often have deeper insights into a channel's mechanics than a sole practitioner, or a single-vertical company.
While platform knowledge is becoming less of a differentiator on highly automated platforms like FB, some channels are so legacy even account reps aren't aware of all functionality, targeting features, etc.
Every edge counts.
Agency Pro #2: systems to iterate quickly
A great creative can 2-3x your ROI, but it's almost impossible to predict performance.
Creative iteration is a real edge these days, and good agencies usually have a wider pool of creative resources.
Agencies will also have insights into tactics that work in other verticals that may be applicable to yours.
You will need a steady flow of new concepts to find new winners as ads fatigue.
Agency Pro #3: mitigate key man risk
As you grow, your internal team should grow.
Using an agency in parallel can fill short-term slack if an employee moves on.
When's the right time to bring on an agency?
step 1: use your internal hire(s) to validate a few channels for growth
step 2: When you hit your stride or plateau, hire an external agency to see if they can improve on what you're already doing
Potential outcomes:
a) Agency can't beat your team's performance:
High five - your team is awesome.
b) Agency finds a more effective strategy:
Your internal team applies it to new channels and you accelerate growth further.
Win or Win.
Thanks for reading.
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I publish 1-2 threads/week on marketing, startups and human behavior. Follow me at @bbourque if that's your thing.