There is no concept of a rule currently in the ANSI standard. SQL Server has featured rules for many releases. Of the two methods, rules are more flexible. They are useful when a constraint exists between two columns of the same table. Favor the use of rules over check constraints when either would suffice and portability is not essential.
Rules are reusable and more flexible.
Favor the use of check constraints when a comparison is required between two columns of the same table. This can not be achieved using a rule. It is possible for both a rule and a check constraint to be defined for a single column. If this occurs, be sure that the two are compatible. Unique Constraints SQL Server 10 also supports unique constraints.