Zac is great fun. So is Rek'Sai too. But Zac is more interesting for longer. Rek'Sai starts to get boring.
Rek'Sai is very popular and strong currently. Fairly fast and sustainable clear. Optimal runes are more mainstream (attack damage/attackspeed). Potent ganks. Pretty straightforward mechanically. On the radar for some minor nerf
Zac jungle clear efficiency is highly impacted by runes in my opinion. I am running attack speed marks, armor quints, scaling hp seals, and some flat cooldown reduction glyphs. This grants you more armor early, the attack speed needed which helps you kill the big mob at each camp, and some hp to scale up with cinderhulk. The cooldown reduction glyphs are important in my opinion as Zac drops blobs with abilities; when he picks up a blob the cooldown of his W drops, so he sort of gets extra effectiveness with cooldown reduction. More abilities, more blobs, more reductions. But when you are playing Zac correctly his clear speed is very good the entire game, and he remains very healthy after the first clear. And his ganks are amazingly fun as you rank up his slingshot.