One note about the marine cable. It is finely stranded, much more finely than what I think was some 8 gauge THHN cable I bought previously at Home Depot for my truck. That makes it pretty flexible. I would guess that the SO cable is pretty flexible too since it is kind of a power cord type application.
I vaguely recall reading in my research that electricity flows on the outside of the conductors or something so I wonder if the fine strands are better for that? I suppose there may be a purpose to choosing heavy strands over fine but maybe not and it is just a quality thing. [EDIT: upon further research it seems it may just have more to do with how flexible the cable is.] Maybe there is also a difference between AC and DC in how it travels on the wire and if fine vs thick strands matters. Of course, household wiring is generally not stranded for common romex runs and that is AC. [EDIT: I researched this concept of power flowing on the outside a bit more and it is called the "skin effect" and applies more to AC than DC]
The welding cable I got for the 12 volt
run between the solar controller and the battery bank is also finely stranded.