M:M Table Design - Use of PK

I have a many-to-many table that contains id's from 2 related tables.

For creating a primary key on this table, is it better to create another integer column and have that as a surrogate PK?

Or should I create the PK on both id columns?

What are the benefits of either method?

Here is the table design with the surrogate key (CodeGroupMappingID)

CREATE TABLE [dbo].[CodeGroupMappings](    [CodeGroupMappingID] [int] IDENTITY(1,1) NOT NULL,    [CodeID] [int] NOT NULL,    [GroupID] [int] NOT NULL,     CONSTRAINT [PK_CodeGroupMappings_CodeGroupMappingID] PRIMARY KEY CLUSTERED ([CodeGroupMappingID] ASC))