How is the correct way to play the QGD Janowski ?

Which is the correct way to play the QGD Janowski? In some different places I see people playing/teaching in different ways like: 

1.d4 d5 2.c4 e6 3.Nf3 a6 4.Nc3 dxc4 

Other way: 

1.d4 d5 2.c4 e6 3.Nf3 Nf6 4.Nc3 a6 5.e3 (or Bg5) dxc4 

Which one is correct?

Thank you!

@1
Both are playable.
Top players prefer to insert ...Nf6.

It's just a transposition, and 5e3 dxc4 6Bc4 c5 if black choses to play that way is transposing into a Queen's Gambit accepted.

There are more extreme ways of playing delaying Nf6. For example after 1.d4 d5 2c4 e6 3Nc3 a6 4pxp pxp 5Nf3 Carlsen likes to play 5...h6 which is kinda of a funny way of trolling how harmless he thinks exchange variations are.  It does avoid 5...Nf6 6Bg5 if you think it is worth avoiding.