Difference between Lookup Filters and Validation Rules in Salesforce
Validation rules and lookup filters both are very similar, but offer different advantages.
Use a lookup filter:
You want to improve user efficiency by limiting the number of available options in a lookup search dialog.
You want to improve user efficiency by automating filters on lookup search dialogs that your users manually set.
Use a validation rule:
You’re close to the maximum number of lookup filters allowed.
You must implement a complex business rule that requires you to use a formula. Formulas can reference fields that basic filter criteria can’t reference, such as fields on the parent of the source object. Formulas can also use functions. For example, use ISNEW if the rule should only apply on record creation, or ISCHANGED if the rule should apply when a field changes.