Hey Kelly,
Firstly, welcome to team purple 
Great question to be asking - best practice is absolutely to use global forms wherever feasible as it ensures consistency, accurate capture of all your required data points etc (especially important for managing data privacy requirements!). And, of course, it's more scalable. Global forms should live in Design Studio and I'd recommend naming them very clearly if they have distinct use cases (e.g., if you requirements for a newsletter sign up form are different from a webinar registration form).
Personally I will typically only create a local form if it's a use case which I can be fairly certain is a once-off, and if the form requires specific unique field set up.
One thing to be aware of - if you're using workspaces - while you can share landing page templates and email templates across workspaces, you cannot share global forms. If memory serves me right, I don't think you can clone them across workspaces either. So if two workspaces share the same need for a form, they'll both need a version of that form created in their own workspace.
Ankit is correct that if you edit a global form that's used by multiple landing pages it may require you to re-approve each landing page that utilises that form for changes to take effect (there are some conditions here around how the form is embedded though). But I don't see this as any significant inconvenience, it's pretty easy to view all the landing pages in design studio and re-approve them quickly.
A pretty key example of where using global forms has significant benefits over local forms: GDPR roll out. It's much easier to roll out updates to meet GDPR compliance on one or two global forms than 50+ local forms
Long, but hopefully helps give some food for thought!
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