Agency will split their employee's time between few customers, in-house has more time to focus only on you but low motivation of your in-house team can hurt their performance even tho they're working on your project all day, every day.

With in-house, whoever you hire to work and to manage your work, is the only one who will dictate your success, he doesn't have a large team to consult with or different opinions to hear but if he's well connected or participating in high quality communities, he can consult with other over there.

Also when changes are made on Google's side - he's more limited with the statistics he can get about what got affected and how, but that's mainly an advantage if the agency is having a lot of customers in your field.

So at the end it's up to the quality of the team and not where they're sitting.

Money-wise you need to spend well on any of them, high quality agency that needs to spend a lot of time on your project wouldn't do it for a low price, and a quality and experience in-house won't take low salary. Agencies may get discounts for their expenses due to high volume but that's rarely get to their customers.