Hi. What is the best practice to test smart contract? mandos or interaction snippets?

Start with some unit tests in Rust. Rust integration tests that generate Mandos coming soon. Until then, Mandos by hand. Finally, some snippets. Snippets are hard, so we usually leave then until the end

If I had to choose one it would be mandos :) But ultimately you will want both. Mandos for quick testing during development and a final on-chain test to make sure everything works as it should and you got the right gas, etc.