Personally I lean toward VRay Dirt, if you have access to VRay. If I don't have VRay, then I would use scanline rendering with a custom light dome set up. There are a bunch of light dome scripts to help with that process.
I tend to avoid Mental Ray because it has a lot of issues outputting normal maps and jumping between render engines for different maps is a workflow nightmare. I pick one that works for all maps (usually Vray) and stick with it.
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Laurens Corijn goes over some of the different methods of baking AO.
http://www.laurenscorijn.com/articles/ambient-occlusion-baking
Personally I lean toward VRay Dirt, if you have access to VRay. If I don't have VRay, then I would use scanline rendering with a custom light dome set up. There are a bunch of light dome scripts to help with that process.
I tend to avoid Mental Ray because it has a lot of issues outputting normal maps and jumping between render engines for different maps is a workflow nightmare. I pick one that works for all maps (usually Vray) and stick with it.
http://wiki.polycount.com/wiki/Ambient_occlusion_map