You have a law firm you're wanting to advertise via PPC ads but you're not sure if you should run Facebook Ads, Google Ads or both? Where is your ad dollar going to be best spent for your law firm?
You could test both but if you were my client, I'd recommend just going all in on Google Ads and ditching Facebook Ads. Facebook Ads is great for 'impulse purchase' type products, which a law firm is definitely not.
You want to reach people who are actively looking for a law firm via Google search because they need your help right now. They're further down the sales funnel and closer to taking action.
You're not going to find that on Facebook. You may get lucky and get a customer or 2 but by and large it'll probably be a waste of your time.
So then you must wonder, should you do Google Display Ads or Youtube Ads in addition to Search Ads? For limited budgets, I'd say no, stick with Search Ads for similar reasons; Display Ads and YouTube Ads are better suited for 'impulse purchase' type products because it is disruptive advertising, just like Facebook. You're not reaching people at the point of intent when they're ready to take action.