Hashgraph vs Xtrabytes



I’m trying to compare Hashgraph and Xtrabytes.
To me these two seem to be the most scalable smart contract platforms right now or did I forget a platform? Iota doesn’t offer smart contracts yet. Neo and Lisk might also be candidates that rank high in this comparison.

Anyway, the criticism of Hashgraph that I found is that the protocol is patented and it hasn’t proved itself in the wild.

Does anyone has more background knowledge how hashgraph and xtrabytes compare in terms of security, usability (smart contract development), transaction costs, flexibility (use case support), extensibility, maturity and suitability for scenarios with low-resource IoT devices?
Is there any technology with similar capabilities like hashgraph and xtrabytes that I should also consider?

Thank you.