Looks to me like it has something to do with your sky texture. Can you upload a Blend file? As far as the shading goes they both look the same to me, the AO pass is just multiplying the effect of ray trans. You could always download a CVS build and composite with nodes using a seperate pass for AO which will allow you to tweak it’s values without having to re-render. Post processing this way is extremely fast because you’re bassically just tweaking the luma or chroma values of the pass .
Something else you might try (also with a CVS build) is setting the offending strutts on a seperate layer from the ones that you like and flagging their visibility within the scene context on the renderlayers tab but not within the layer context on that same tab. Then set up another render layer where you have your offenders flagged in the layer context but not the non offenders. Render both layers with an AO pass and be sure to link them together with a z-combine node. Now you can tweak your AO pass for the good guys and the bad guys in two seperate post processes and attain a much greater degree of controll. Situations like yours are exactly why the devs coded the new pipeline. CONTROLL!.
The two monkeys in the image were set up exactly as described above. AO is the only lighting used in the scene, yet you can clearly see the difference in the girls due to tweaking of the seperate AO passes. As far as adjusting the strength of the pass in post, it updated IMMEDIATELY, as fast as I blinked. Beats the heck out of having to re render many times to get what you’re looking for, especially when AO is involved. If I’m not mistaken you can animate this value via a color ramp and a time node though I haven’t fiddled with that yet. In v 2.44 we will most likely be able to animate values output from the blue sockets also, but even the static values in the upcoming build are insanely cool.
Also, here’s a link to the .blend:
The soften filter that I added to the z outputs of each renderlayer is to eliminate the few artifacts that crop up when using z-combine. The devs have added masking to the z-passes in order to minimize the extreme aliasing associated with the pass but it’s not perfect. Filtering this way makes it smooth as a baby’s bottom.