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/