I have been following both projects with great interest, and they share similar characteristics, such as creating an economy that rewards developers and users.
Although blockstack is technically a light-weight blockchain that piggy-backs a blockchain, and Kin is a smart contract that has the Kin Rewards Engine - Are the two systems basically competitors?
Blockstack is quite different to QTUM and Ethereum. These two are blockchains that are ‘heavy’ and inherently slow in their architecture (for now).
As I understand it, Blockstack is a virtual blockchain that can run on top of a blockchain like Bitcoin or Ethereum. The advantage of this is that many of its processes and data storage will run on layers outside of the blockchain, and only settlement (i think) and identity will be stored on the underlying blockchain.
This makes it much faster, scale-able, easy to update without forking, and easy to port across chains without any issues.
It also has rewards built in to foster use for developers and users of their apps alike.