NOT EXISTS or LEFT JOIN which one is better?

[AI Rumman <rummandba@xxxxxxxxx>]

I can write a query to solve my requirement in any of the followings :-

1.

select *

from a

where NOT EXISTS

(

select 1

from b

where a.id = b.id)

union all

select *

from b

2.

select 

(

case when b.id is not null then

   b.id

   else

   a.id

) as id

from a

left join b

  on a.id = b.id

Any one please tell me which one is better?