Great question! Nothing better than Ethereum's own documentation for explaining this: https://ethereum.org/en/developers/docs/scaling/plasma/ ​ The short tl;dr is plasma chains inherent the same base security as Ethereum mainnet, while sidechains do not, sidechains (like Polygon Matic) are essentially independent blockchains that just happen to be associated with Ethereum and have an easy way to bridge funds and assets between the two chains. ​ As a consequence, plasma chains are generally far more secure and a more technologically sound way of scaling Ethereum versus sidechains :). ​ To quote from the Ethereum docs: ​

A plasma chain is a separate blockchain that is anchored to the main Ethereum chain, and uses fraud proofs (like optimistic rollups) to arbitrate disputes.

​ And as for the difference between plasma chains and sidechains: ​

A sidechain is an independently operated blockchain connected to Ethereum Mainnet via a two-way bridge. Bridges allow users to exchange tokens between the two blockchains to transact on the sidechain, reducing congestion on Ethereum Mainnet and improving scalability.

Sidechains use a separate consensus mechanism and are typically much smaller than Ethereum Mainnet. As a result, bridging assets to these chains involves increased risk; given the lack of security guarantees inherited from Ethereum Mainnet in the sidechain model, users risk the loss of funds in an attack on the sidechain.

Conversely, plasma chains derive their security from Mainnet. This makes them measurably more secure than sidechains. Both sidechains and plasma chains can have different consensus protocols, but the difference is that plasma chains publish Merkle roots for each block on Ethereum Mainnet. Block roots are small pieces of information we can use to verify information about transactions that happen on a plasma chain. If an attack happens on a plasma chain, users can safely withdraw their funds back to Mainnet using the appropriate proofs.

​ Hope that's helpful! Thanks for the question Jeshua!