When we go for pre-validations and when we can go for pre-stage?

 Pre-validation :- Pre-validation stage executes outside Database Transaction which will not rollback the entire operations written in the plugin if there is any runtime error occurs in one operation. This provides an opportunity to include logic to cancel the operation before the database transaction.

Pre-operation :- Pre-Operation stage executes inside database transaction due to which any runtime error occurs in a single operation in the plugin will rollback entire operations which are part of the plugin.
If you want to change any values for an entity included in the message, you should do it here.