For what purpose, simple grounding (e.g. DC circuit returns) or overall? With AC and particularly RF you need to consider skin effect. For the simple return path for your battery it's doesn't matter, the largest cross section (or equivalent thereof) is what you want regardless of construction. The reason straps and high count stranded cables are used is practical, they flex and are easy to route. To some extent using them also helps reduce EMI as they work better as conductors than antennas for the pulses created by the alternator, ignition and the digital noise produced by all the computers in cars. But in general it's not important. Now for RF paths, braided cable is a necessity as you've probably seen mentioned often. It's absolutely critical to having any usable HF in a mobile situation.
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