For a casino player the comparison has practical effects. A balance kept in bitcoin rises and falls with its price between sessions, while a USDT balance aims to keep its everyday value but depends on its issuer. USDT runs on several networks, so the network chosen at the cashier must match the one used to send. Tokens on Ethereum need ETH in the same wallet to pay gas. The law that governs gambling with crypto is the player's local law, whatever licence the casino holds.
Bitcoin vs Ethereum comes down to purpose and design. Bitcoin was built mainly to send and store value, and its network does little beyond that. Ethereum is a platform that runs self executing contracts, small programs that execute on the chain, and thousands of tokens are issued on top of it, including one version of USDT. Its own coin, ether or ETH, pays the gas for every transaction there, so a wallet holding only tokens still needs some ETH before anything can move. The two coins have separate networks, prices and address styles.
Bitcoin vs USDT is a comparison between two different jobs. Bitcoin has no issuer; new coins come from the network's own rules, and its price is set by buyers and sellers, so it can rise or fall sharply within a day. USDT is a stablecoin issued by a company that aims to keep each token equal to one unit of a traditional currency, backed by reserves it reports holding. Its steadiness rests on trust in that issuer, which can also freeze specific addresses. Neither is better in general; each suits a different purpose.