Just to give you a quick background: During my career, I have worked in start up, mid-size organization and with one of the biggest employers in Finland. Also, I have been in-house recruiter, consultant and also something in-between. That's why I have quite a good idea, what different positions have to offer to your career development.
You should ask yourself one question
In my opinion, you should ask yourself on question: do you want to work in a support function (the company's core business is something else than TA) or to be the part of the company that is bringing in the revenue (your expertise will be what the company is selling, and you will be bringing in the money).
I have learned, that some people prefer one and some the other. Either way, TA is, and should be a strategic partner to the business, but the trick is that is it you or someone else bringing in the revenue.
Here are some pro's and con's in both:
Inhouse role: One company to work with & you can act somewhat in the background
+ Your have your own familiar team to work with in a daily basis
+ You get to know people you work with in a really deep level
+ You are there for a long time so you get to see how the company - and your own field of responsibility - develops
+ You get to learn about one company and their business in a really deep level
+ You get to learn the company's target talent groups really well (e.g. data scientists with really specific knowledge)
-You are in your own bubble and it's harder to see what's happening within the field (socialising with colleagues in the field outside of your company becomes crucial)
-After a while, it is harder to recognise your company's unique selling points
-Very often same or similar profiles you work with
-You are stuck with company's problems and can't "escape" them
-You are selling yourself and your work internally
-You are fighting over TA budgets and how much you get to spend
Agency: Many companies to work with and pressure to bring money
+You have a really good view for what is happening within the TA industry
+You usually have a lot of colleagues from the TA industry to brainstorm with
+You definitely won't get bored because of many different customers
+Different types of challenges you get to solve
+You recognise companies bottlenecks pretty easily
+ Because of the wide range of customers, you understand the unique selling points quickly
-Many different companies and many different roles to understand
-Balancing between development work and consulting work
-Limited opportunities to make decisions in customer companies TA
-Limited access to company information
-Different level of pressure when it comes to succeeding (you are bringing in the money, remember)
For me, in-house work has given more learnings considering the company's business & industry and and working in an agency has developed me the most as a talent acquisition professional. Still always, it is about selling your thoughts and trying to do your best job to find and hire the best talent.
To conclude, the job itself is similar and, from my experience, either way you choose, you'll learn something valuable for your career.