Plasma is a different solution to solve scalability issues and is designed to be used in sidechains (also called childchains) which can have their own consensus mechanism. The tokens are being locked in a smart contract on the mainchain and issued on the sidechain. Of course you have more flexibility on a sidechain and can interact with smart contracts an indefinite amount of users like on the mainchain. But still all transactions are visible to the public.
Additionally you have some drawbacks if you want to leave the sidechain and get your tokens back. In Plasma there regular snapshots being made to reflect the state on the sidechain on the contract of the mainchain. When trying to get your assets back from the sidechain you need to wait a certain time window.
A state channel normally is only established between two parties and those parties can (at least in the powerful æternity implementation) define certain conditions for opening/closing the channels. It is even possible to execute smart contract logic within those state channels. Also returning assets from a state channel can be way quicker (or immediate if both parties agree) and as long there is no dispute all activities within the state channel remain private.
With the plan to introduce virtual state channels we will have even more flexibility regarding off-chain transactions. If virtual state channels are being introduced A will be able to open channels to a party C if A has an open channel with B and B has an open channel with C. The funds would then be locked in the channels of A <-> B and B <-> C and even if B would go offline A <-> C would still be able to transfer funds within the virtual state channel.
But if you want too see sth. like Plasma on æternity I think it should be possible to implemented this in a similar way the folks on Ethereum did.