The Cactus V6 system is great if you have legacy TTL flashes in the Canon/Nikon/Pentax systems. Otherwise, you have to get the Cactus RF60 as your off-camera flash as it's the only manual flash the V6s can give you remote power control on.
YN-560III/IV is a cheaper unit, and gives you all the same function (remote power/zoom) if you use a YN-560-TX as your dedicated transmitter, and, like the RF60, has the bonus of a built-in receiver, so you don't have to remember to bring along the trigger and batteries for same. A cheaper unit matters if you plan on buying multiples.
But Yongnuo isn't really made for heavy-duty pro usage, so if that's your usage case, I'd say go for something more robust like a
LumoPro LP180 and give up remote power control, or maybe consider a more robust, expandable triggering solution, say
Phottix,
RadioPopper, or
PocketWizard triggers--where the manual and TTL triggers can inter-operate with each other, so integrating additional types of lights, such as studio strobes, becomes less of a hassle. Or, pick a triggering system that can control the power level on your studio strobes (e.g.,
Cybersyncs with AlienBees, or FT-16 with Godox
Wistros or Vings).
I'm a hobbyist shooter who only occasionally does off-camera lighting, and I own a lot of Canon eTTL-II gear. I use my YN-622C transceivers, my 580EXII and YN-568EX, and a YN-622C-TX with my Canons (where I have full everything-- TTL, HSS, remote power, settings, groups on/off, etc.), and my micro four-thirds GX-7 and X100T (where I have remote manual power/zoom control and groups on/off).