Gragas and Zac, one in the same?

During the season I starting playing Gragas because he seemed fun. Being a jungle main for most of the season, I started playing other champions that also fit the role of jungle/tank. So I bought Zac, and immediately had no issue playing him. The more I practiced and played Zac, I started to realize that Zac is Gragas but with higher cool downs and a much more forgiving ultimate. If you look at each skill individually, you'll notice small difference, but see a lot of the resemblance. Q is a ranged slow, one on a timer, one not. W is more utility as it's a self buff in a sense. W for Zac is a source of damage, but also the fastest cool down in his kit which restores health. Gragas' W gives him defense buffs, and also does damage. E are both engage/disengage tools from range. These are their initiate lockdowns, both knocking up and stunning. R is a knock up/back, Gragas takes more skill to use, while Zac can just hit and then click where he needs to go. I think the biggest reason Gragas is picked over Zac is strictly for the cool downs on his skills. Zac's engage is double the cool down of Gragas', which makes Gragas a better pick for professionals. But if anybody else was wondering why Zac was picked during Worlds after Gragas was disabled, this could potentially help you understand. Does anybody else agree with this comparison?