There are many fit and styles of suits now these days in the fashion market. Slim fit, Regular fit, Modern fit, Classic fit, and etc...Make sure you read the description and the fit of the suit you are buying.
If you want a slim fit suit, don't buy a regular fit suit and go one size down.
If you want regular fit suit, don't buy a slim fit suit and go a size up.
In some cases you can do that, but in reality a slim fit suit should fit slim and a regular fit suit should fit regular. if you change the size because of the fit, you defeat the purpose of the style.