What is the difference between Unique Key and Index with IsUnique=Yes?

Question

I have a table with a primary key, but I want two other columns to be constrained so the combination of the two is guaranteed always to be unique.

(a dumb example: In a

Answer 1

I do not think there is any difference between them but using unique index , we can have two benefits , as the column is already unique and also had the index on it so i gonna be more faster to search . So using unique index is more benefit.