Both Zac and Maokai are strong junglers, so which one you choose will depend on your personal playstyle.
Zac is a high mobility tank jungler who can jump long distances and disrupt enemy positioning with his E (Slingshot) ability. His W (Unstable Matter) also allows him to clear minion camps quickly and effectively. He's also incredibly tanky, meaning he can engage in teamfights without fear of being easily killed.
Maokai, on the other hand, is a tanky disruptor that excels at controlling the battlefield with his CC abilities. His Q (Bramble Smash) can stun enemies and knock them back, and his W (Twisted Advance) allows him to dash to enemies or wards and root them in place. His ultimate is a large AoE knockup that can turn the tide of a teamfight. He's also very difficult to kill, thanks to his passive healing.
Maokai is generally easier to pick up and play than Zac, as his abilities are fairly straightforward. However, Zac's mobility and disruption potential can make him a powerful jungler in the right hands. It'll ultimately depend on which style of play you prefer, but both champs can be very effective junglers when played correctly.