Is it better to transfer BTC or USDT?

I'm relatively new to the crypto world and have some funds that I'm considering transferring. I've heard of both BTC and USDT, but I'm not sure which one to choose for my transaction. Can you help me understand the pros and cons of transferring each? I'm mainly concerned about transaction fees, speed, and security. Additionally, I'd like to know which one has better liquidity and acceptance among crypto exchanges and merchants. Thank you for your assistance!