Tether is a risk but the team has promised to keep improving how it operates and have an actual audit soon. There are not much better option for traders as alternative to USDT, there USDC but it doesn't even trade with 10% of what USDT does.
No differences between them all. They are all minted by their teams and even claimed to be audited but rather no proofs that all the issued tokens after the audit would have USD cover reserve.