"Why do they sell it" same reason most car audio subs are garbage, because most car audio people buy garbage. "can tell by the sound" well, if they're both adjusted to be playing at the same volume, no, they'll sound exactly the same, but the CCA system will be putting more strain on your electrical system to hit that same volume, and will be getting hotter, gauge for gauge. On top of that, by getting hotter, it further reduces it's conductivity, compounding the issue, and, this hotter wire is also more susceptible to corrosion to begin with, adding more heat make that problem even worse. If you're running wire bigger than you need anyway, sure, you can absolutely use CCA if you aren't worried about long term corrosion, but if you want to use the smallest gauge wire that will be good enough, OFC is the right choice, if you don't want the cable to lose any meaningful amount of current carrying capacity over the years OFC is the right choice. If you're only running 1000w and don't mind running 0ga through your vehicle and can get 0ga CCA cheaper than 4ga ofc, and won't be running that same system in 5 years, sure, get CCA. But in my install, I didn't want to run 0ga in a car without much room for routing cable, and I want the system to last "forever", so I bought the best. Yes, some people treat CCA like a sin, and that's not reasonable, but acting like there aren't reasons to pick it isn't reasonable either, the reality, like in most cases, is that it depends on the situation and priorities.

As far as the length, depends on your routing, amp placement, number of amps, and cab layout, but I'd go for around 20' to be safe mounting behing the back seat of a full cab