I've stayed at both with my SO earlier this year.
There are a lot of other review threads on FT, so definitely search for it.
Personally, Ziva has more food options and a larger number of pools. If you're not allergic to children I'd generally recommend Ziva.
Also, FYI you can actually make reservations at the other hotels (at Zilara, we were given a reservation at Ziva to try different restaurants). We also walked back and forth between the hotels and didn't have any issues with drinks being provided - it's a bit of a hike, but a fun walk along the hotel zone along the beach.
Honestly though - pros and cons of each. Both are very solid all inclusive and I think that just about everyone would have a good time at both.