They’re similar in some areas, such as that they both offer something called zk rollups (aka “zero-knowledge rollups”, which are used to validate data on a blockchain in a way that’s quicker than the more standard proof of stake mechanism). But Loopring has more of a focus on helping decentralised exchanges (along with a rumoured partnership with an NFT marketplace), whereas Polygon allows people to make smart contracts, give loans, while also having capability for decentralised exchanges. But even though Polygon has more use cases, that doesn’t mean the technology is going to prove to be better in the long term.
At the time of writing, Polygon is the 16th most popular coin by market cap, while Loopring is 75th. This makes the latter way more volatile, which can create bigger rewards but is riskier. Plus, given their competitors, it’s fair to say Polygon is way, way more popular right now. It’s like Everton versus Mansfield Town.
Bitcoin 
Ethereum 
Tether 
XRP 
BNB 
USDC 
Solana 
TRON 
Lido Staked Ether 
Figure Heloc 