What's the difference between a Table Scan and a Clustered Index Scan?

Since both a Table Scan and a Clustered Index Scan essentially scan all records in the table, why is a Clustered Index Scan supposedly better?

As an example - what's the performance difference between the following when there are many records?:

declare @temp table(
    SomeColumn varchar(50)
)

insert into @temp
select 'SomeVal'

select * from @temp

-----------------------------

declare @temp table(
    RowID int not null identity(1,1) primary key,
    SomeColumn varchar(50)
)

insert into @temp
select 'SomeVal'

select * from @temp