so the ones that have the ferrite cores are actually lower quality products (generally 28 gauge wiring). The ferrite helps clean up any noise that might occur from close proximity of high gauge wiring. You dont need them on the lower 24 gauge stuff because they dont bleed as bad.
The ferrite is helpful for shorter higher gauge wires. In low gauge wires its not needed, and high gauge long run, it can actually filter the signal, so you'll want to avoid 28 gauge in anything over a 10' run (25 max from the sensible videophile contingent, and some devices might have problems with the signal strength at this distance).
Hopefully that answers your question. If you're buying a 6ft 28gauge, get the ones with the ferrite cores.