catalan vs classical queens gambit

I've had alot of success with the catalan opening recently, I understand the concept behind the queens gambit but have alot of trouble applying it. In the catalan the strategy is farely clear to a point. what are the differences between the two? strengths and weaknesses?

It's the matter of taste.  Notice that Catalan with 3...d5 is the same as QGD with white's kingside fianchetto:

This diagram illustrates the transition QGD --> Catalan. 

The Catalan can actually come from 2 openings.  The QGD or an attempt at the Nimzo-Indian:

1.d4 d5 2.c4 e6 3.Nf3 (this should be played first in the QGD move order, but not the Nimzo-move order in case Black plays ...c6 with no ...Nf6) Nf6 4.g3

1.d4 Nf6 2.c4 e6 3.g3 (...Nf6 and ...e6 are both already played, the main requirement for the Catalan)

The main weakness in the Catalan is the c4-square.  After after Black grabs on c4 and White takes it back with the Queen, Black will often play ...b5 and ...Bb7, and c4 often becomes a soft spot for White.  The main strength is the Bishop on g2.

In the QGD (1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Bg5) the battle is over the tempo.  Black wants to take on c4, but not until White has moved his Bishop to e2 or d3 (doesn't matter which).  He then is looking to break in the center with ...e5 or ...c5.  White's plan depends on what line he plays.  For example, lines where he castles long (Rubinstein - 7.Qc2 instead of 7.Rc1 - or the Exchange Variation with O-O-O), White goes for a Kingside pawn storm.  When he doesn't, his play is predominantly on the Queenside, and particularly the c-file.  Black will usually go for the e-file and a Kingside attack.

I like the Catalan a little better as white than the normal queens gambit lines. The bishop on G2 is a big asset since it either can shut down black's queenside, or black spends many moves blocking the diagonal with knights and/ or pawns. Sometimes, in doing this, black gives up tactical chances where a well placed sacrifice can blow the center open. There are a lot of ways for black to play and more often than not you will reach an endgame in this opening.

The Catalan players to keep an eye on: Giri, Gelfand, Wesley So, Aronian (just a few games) and Kramnik.Ulf Anderrson, Korchnoi, Kasparov, Karpov

Catalan is better option, i could play both systems. 

Looks like d5 players have to be prepared for both QGD and Catalan.  Is there a black defensive setup that could be deployed equally well against either?

I was thinking of Slav or a KID setup for a universal defense against d4 and Catalan.

I have alot of trouble against the slav using the catalan. Unless they take on c4, then I make alot of use of the dark squares with my knights. against a kings indian as long as I can keep it an open game I usually come out on top because of my greater piece play. In the catalan what should I do with my dark square bishop, I find that it can be quite usefull in taking the knight on f6 but that's not always the case. sometimes I'll place it on f4 but find that mostly useless most of the time. thanks for your answers!

What would you call this? A Reti? A Catalan? A QGD Chigorin? A mess?