Well, those aren't written correctly. The correct form would be
--1
select T1.Col1 from T1
where exists
(select * from T2 where T2.Col1 = T1.Col1)
--2
select T1.Col1 from T1
where T1.Col1 in
(select T2.Col1 from T2)
And those two are completely equivalent.
http://sqlinthewild.co.za/index.php/2009/08/17/exists-vs-in/
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability