USDT (Tether) and USDC (Circle) are the two largest dollar-backed stablecoins, together worth hundreds of billions of dollars.
USDC is widely seen as more transparent and regulatory-friendly, with regular attestations of its reserves. USDT is larger and more widely traded but has faced more scrutiny.
USDT dominates trading volume on many exchanges, especially outside the US. USDC is favored in DeFi and by institutions.
Both are available on many blockchains, including Ethereum, Solana and several layer 2 networks, so they are easy to move around.
For most purposes the two are interchangeable. The choice often comes down to which your exchange, app or counterparty prefers.
Both are widely used and generally considered stable. Your specific use case will usually dictate which one you hold.