ZenCash claims to be the most decentralized architecture comes from the fact that they have a higher number of nodes than even Bitcoin, in term of true decentralization though I'd say Bitcoin still have the lead. Compared to Bitshares, I'd say Zencash is an entirely different project. ZenCash goal is to build a private platform for media, money, messages, computation and exchanges etc where Bitshares is (afaik) only a decentralized exchange. Go read ZenCash's new whitepaper:
https://zencash.com/assets/files/Zen-Application-Platform-Whitepaper.pdf

Cheers!