it really depends on what type of pawn structure you want if white plays cxd5. The QGD protects d5 with e6 and the Slav protects d5 with c6. if 1.d4,d5 2.c4,... black has multiple options to defend besides the two you have mentioned including the QG accepted and Chigorin defense. Technically the QGD is actually a family of various variations including the Tarrasch defense with c5.
Slav or Queens Gambit Declined?

good. even studying opening that you wouldn't play expands your chess knowledge. patterns, pawn breaks, identifying tactics, transpositions, positional ideas. learning the QG accepted will take a lot of white players out of their knowledge and comfort zone.

um
slav is more sharp and dynamic (also much more theory required to play it)
normal orthodox queens gambit declined setups and typically more ideas based and less dynamic
example lines from both:
It makes absolutely no difference. Play both of them and discover which one gives you positions that you enjoy playing. Objectively, they're both fine

Queens Gambit Declined as it has more transpositional flexibility . Examples of this include :
- Semi Slav
- Nimzo-Indian
- Ragozin
- Bogo-Indian
- Catalan
- Kings Indian: Yugoslav Attack
- Tarrasch


I see 1. d4 not too often, but I just want to be prepared if it happens. I have seen some londons, and some people who actually play queens gambit. I dont really need this forum anymore though; Im pretty sure I have decided on the Slav, since it fits well with my Caro Kann, and if they try to london me, I have 2. ... Qb6. Also, I of course just like the positions. Might make another forum post thats more detailed than this one.
Hi. My opening repertoire is mostly fine, but I just wanted some opinions on which of the two QG responses I should play: The slav, or the QGD.