RmF vs. Mental Ray Ambient Occlusion

I set up some test renders to get ambient occlusion passes from RmF, by designating "Occlusion Indirect" as a pass in an EXR:

Open up the Render Globals and go to the "Passes" tab of the RenderMan menu:
Click on the triangle next to "Output" and select "Create Output > Custom" and in the new window that opens, right-click and select "Occlusion Indirect" from the dropdown menu:
Be sure to select "OpenEXR (exr)" in the Image Format menu (as in the first image above.)

I rendered with an Environment Light with the samples set to 256, the Diffuse Softness set at its default .950 and the Max Distance set to 4:
I used these settings knowing that I'd received decent results in Mental Ray with its Ambient Occlusion samples set to 32, as in this post. After using Nuke to pull out the Occlusion Indirect channel from the EXR, I got this result, cropped from a 1920x1080 render that took about 3 minutes to calculate:
Needless to say, the results are pretty horrifying. They were very unexpected (and this is after tweaking the parameters until I set them to the numbers shown above) and very much useless. The windows are gray because the windows' shader was partially transparent, and there is a surface shader mapped to geometry directly behind the windows. In effect, the windows yield a perfectly flat gray; my workaround ended up requiring me to assign an opaque Lambert to the windows. However, the extremely black pockets next to the staircases and behind the gutters made the images absolutely unusable.

I rendered the same scene in Maya 2009, using Mental Ray and assigning an ambient occlusion shader with 64 samples, and got a render in about 5 minutes, which is also cropped here:
The image came out as expected, so as of now, I'm leaning towards rendering the ambient occlusion pass with Mental Ray, with the other passes (e.g. diffuse, specular, shadows, etc.) from Renderman.