Usually it's both - the scene bake won't take occlusion from a high res mesh, but the individual bake won't take occlusion from the scene surroundings.
They'll get combined into a final AO value in some way.
That depends on the quality and performance of your rendering solution. What's your target engine? (Assuming you are talking about realtime rendering.) Normally you'd want to have AO to fake contact shadows anyway (ideally only for the ambient part of the light). AO maps on the other hand can give you more detailed control and AO at no (baked into the diffuse) or little (separate map) cost at render time, though the former can look funky as it allows for no control in regards to the lighting situation. AO maps can also be useful as an utility map, so chances are, you'll be creating them anyway. To give a general answer, you probably want to use both, leaning towards just an AO map for really simple rendering solutions and just an AO pass with increasing quality of the rendering solution and a combination in between (and potentially no AO at all for a "full" rendering solution).
If you do an offline rendering with a highpoly the pass is all you need but for any game or realtime engines you need that AO map for the details, no realtime AO will get you the fine detail that you need in nearly all models. If you use a baked model in a offline renderer you will also need the AO map of course as it can't possibly get the shadowing in the details that do not exist in geometry
What are you rendering? Lowpolies? If you are rendering a HP still, it's an artistic or practical decision on your part, as Arnold being a full blown renderer technically doesn't need any AO at all.
Highpolies. So Arnold does not need AO to achieve fully realistic images?
Arnold is an unbiased brute force pathtracer and 'AO' is resolved as part of the render. In other(older) offline and non brute force raytracing hacks like light cache and irradiance mapping are used to add the AO effect. Of course this doesn't drop you from rendering an AO AOV pass and comping in post.
Arnold is actually more realistic because it doesn't add AO - which isn't really a thing in real life.
If it's possible in Arnold, I would render AO as a separate pass and add it in your compositor, that way you can adjust the strength or decide to not use it at all, without rendering the whole thing again.
Replies
Normally you'd want to have AO to fake contact shadows anyway (ideally only for the ambient part of the light).
AO maps on the other hand can give you more detailed control and AO at no (baked into the diffuse) or little (separate map) cost at render time, though the former can look funky as it allows for no control in regards to the lighting situation. AO maps can also be useful as an utility map, so chances are, you'll be creating them anyway.
To give a general answer, you probably want to use both, leaning towards just an AO map for really simple rendering solutions and just an AO pass with increasing quality of the rendering solution and a combination in between (and potentially no AO at all for a "full" rendering solution).
I understand that AO pass should be enough
If you are rendering a HP still, it's an artistic or practical decision on your part, as Arnold being a full blown renderer technically doesn't need any AO at all.