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Theoretically,,,,they should all be the same potential. In actually there is some resistance in the wire and at each electrical connection. If you have good clean and tight connections, you should never really notice any difference between the battery, frame or engine.
The absolute best ground would be directly at the battery terminal. Attaching a wire to the battery terminal isn't very practical but you can remove the small black wire on the negative battery cable from ground stud G-104/108, cut the eyelet off of it and solder on three black wires. One of the wires you would solder on another eyelet and reattach it to G-104/108 ground terminal stud. The other two can be used as aux grounds wire that come directly off the battery.
Since this post is about grounds,i would like to ask a question,but dont want to hijack anyones thread.
How many and where are the engine compartment grounds?
Take a look at my post "IMPORTANT ELECTRICAL INFORMATION (LONG) in the stickey section at the top of the C5 Technical section. The grounds are covered in great detail there.