QGD vs Semi-Slav

I've been paying the King's Indian and the Grunfeld but I've decided to learn a more solid opening to respond to 1. d4. I've been looking at playing the Queen's Gambit Declined or the Semi-Slav. Which one should I learn? Currently I'm rated nearly 2100, I like to active play in my games and I do like chances for Black to win. Which one would you prefer and why?

I know it's not one of your options but I prefer the Slav. The pawn on e6 blocks in the bishop in the other ones.

Looks like Semi-Slav is the choice based on your preference. It can lead to wild positions with lots of tactics and theory.

But I don't know about diving head first into Semi-Slav if you have no foundation on either QGD or Slav.

White can always deviate from the theoretical mainlines, where understanding of typical QGD or Slav structure and plans will be useful and memorized forcing variations of tactics will be useless.

<But I don't know about diving head first into Semi-Slav if you have no foundation on either QGD or Slav.>

So if you're recommending learning the QGD (I don't care for the Slav to be honest), would it be better for me to just learn that and not worry about the Semi-Slav? That way I would learn just one opening instead on 2. Or, are they so similar that it wouldn't be difficult for me at all to learn both...

They are not similar at all.  Both sides have moved their d-pawn 2 squares and made a couple of other pawn moves.  That's about all they have in common!

Also, if you are going to try to play the Semi-Slav, you must know one of the following 3 as well:

Marshall Gambit - If you do it via the move order 1.d4 d5 2.c4 e6 3.Nc3 c6, intending 4...Nf6, then you must be able to deal with 4.e4, the Marshall Gambit

Queen's Gambit Declined - Either the Orthodox, Tartakower, Lasker, or something of that nature.  After 1.d4 d5 2.c4 e6 3.Nc3 Nf6, intending 4...c6, White can play 4.Bg5! instead of 4.Nf3, and now 4...c6 is premature with White having not played Nf3.  Black needs to go into QGD lines, like 4...Be7 or 4...Nbd7.

Exchange Slav - After 1.d4 d5 2.c4 c6 (intending 3...Nf6 and 4...e6), Black must be willing to deal with an exchange on move 3, leading to the exchange slav.

So depending on the move order you choose, you would need to know more than just the highly theoretical Semi-Slav.

I personally like the Queen's Gambit Declined more for black because I like the Meran Variation against the Semi-Slav for white... that may be just me though :)

Yeah I think ThrillerFan said it nicely.

The annoying thing about Semi-Slav is that there is no independant way of reaching that opening. You have to "borrow" another opening to reach Semi-Slav, be it Nimzo move order, Slav move order, or Triangle move order, or if White, for some reason plays 4.Nf3, then QGD move order.

And that means you have to learn what to do when White doesn't allow you to play Semi-Slav.

So I guess Semi-Slav is kind of like Nimzo-Indian in that, you have to have another opening ready in case White doesn't allow it.