"union" vs. left join

Good morning,

Would it be possible to know that whether "union" is quicker than "left 
join" in a general sense?

Queries:
========================
(1) union
select a.*, b.value
from      a
left join b   union
select a.*, c.value
from      a
left join c


(2) left join
select distinct a.*,  case when ... then     b.value  else     c.value
from      a
left join b
left join c


Will (1) be more efficient than (2) or vice versa? Or it really depends 
on data


Thanks a lot!
Lu Ying