Unless there's a "situation" that I'm not aware of from your pix, the actual chassis will conduct more current than the bus bar - steel isn't as good of a conductor per cross section as aluminum, but there's a lot more of it (I'm assuming). With that said, the real issue with grounding high current loads to the chassis is making a GOOD high current ground - AND - keeping said ground free of corrosion, which can be problematic and its a single point of failure. That's why I have both a chassis ground AND a direct to negative cable on my Jeep winch.
So my gut says if you feel you can make a better connection to the bus bar than the chassis, by all means go with the bus bar - and you won't hurt anything in any event. You'll still have the chassis ground in addition. Either way, make a good ground stackup with star washers, flat washers, and keep it clean and dry.