Ziggs overall, in my opinion is easier to use in teamfight. Vel koz r can be stopped very easy even by ziggs w. In late game Ziggs can nuke 1v5, vel koz can too but its much harder to land your all skills with ultimate, everyone has to stay in 1 lane while with ziggs u can throw almost 1.5k dmg ult in late game and your other aoe abilities, thats why ziggs has an advantage there. If enemy has tanky champions like nasus,garen,darius take vel koz to melt them down with your true dmg, they will just cry a river when they will meet u lmao.
Vel koz > Ziggs Better push early, less mana hungry at early game.
Ziggs >vel koz tower pushing
Vel koz> Ziggs - Better in melting tanky targets,
Ziggs> Vel koz - overall can deal more dmg in teamfight and faster with lower risk.
vel koz> Ziggs - in my opinion better early.
Vel koz=> Ziggs mid game , a little advantage of vel koz, but it is dissapearing fast.
Ziggs> Vel koz - better late.
Overall both of them are used, in the different games and situations. They have advantages and weakness. I cant really tell u who is harder to play , i play both of them, and i can say that with vel koz q is easier to poke someone than with ziggs. But in teamfights its much harder to find a good position to press r as velkoz because u can be chased by assassin or tank, while ziggs just pop off everything he has and goes away.