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 particular addresses. Neither is better in general; each suits a different purpose.
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 the casino's own licence says.
Is bitcoin legal? The answer depends on the country, and no single rule applies everywhere. Some countries regulate buying, holding and exchanging it, some restrict how banks or businesses may deal with it, and a few ban it outright or ban key uses of it. Tax rules differ just as widely, and many places treat selling or swapping coins as something to declare. Rules also change over time, and a website that opens in a browser proves nothing about whether using it is permitted. Local rules are the only reliable answer.