Exchange ETH to USDT

Hi guys, I need to exchange the ETH that I have in my Trezor beta wallet to USDT, which is the safest and cheapest way to do it, so that the USDT can stay inside my Trezor wallet?
Besides, I want to know if the Trezor beta wallet is as secure as the standard Trezor wallet, I mean the wallet in which tokens that are not ERc20 are stored.
Thank you!