Grunfeld or Benoni

I was considering learning a new opening against 1. d4 as black. But I've yet to decide whether it would be better to learn the Grunfeld Defence or the Modern Benoni. Btw I'm 1900 USCF, if rating should be considered. 

I prefer semi-closed or closed games

I don't know much about these openings, but I know that there is one line in the Modern Benoni that is said to almost refute it. Start by checking that line out. If Black has no reasonable way to equalize there is no reason to play that opening at all, at your level. Your opponents might very well know the winning line.

dozeoff is probably referring to the Taimanov Variation/Flick-Knife Attack. 1.d4 Nf6 2.c4 c5 3.d5 e6 4.Nc3 exd5 5.cxd5 d6 6.e4 g6 7.f4 Bg7 8.Bb5+.

Black has to play accurately, or else White will march his pawns up the board and stomp all over him, but by no means is it a refutation. In a few months of playing the Modern Benoni exclusively against d4, I can't recall facing 7.f4 even once, so I wouldn't expect to run into anyone so booked up that they'll blow you off the board. Nevertheless, some players prefer to play a Nimzo move order (1.d4 Nf6 2.c4 e6), playing the Nimzo or QGD against 3.Nc3 and only playing the benoni if White plays 3.Nf3 (since f4 is no longer possible)

I was once a Benoni ( modern ) fan myself and played it ( and the KID ) for years against 1 d4 . Now I still play it , but rarely and only when my opponent is under 2200 ( otb ) and plays the 3 Nf3 move order. I do like to avoid the taimanov line but also the penrose/Tal attack . 

Both are taxing and sharp openings.

Ben-Oni is easier to play, and should provide more winning chances at U2200 level.

Grünfeld is more difficult but could bring you higher in terms of chess ability.

If you already have the Nimzo or the QGD in your repertoire, it would make good sense to play the Ben-Oni with the move-order 1.d4 Nf6 2.c4 e6 3.Nf3 c5, as Reb mentioned above.

Thanks guys. I've played the nimzo a lot with white but Ill still have to brush up with Black theory. And I think I'll give the Benoni a try

I think the Nimzo and the Ben-oni are a great combination. You can use the move order 1...Nf6 2...e6 and leave your opponent in the dark for a while.  Having both in your repertoire works well if your opponent plays the English, he will have a hard time playing d4 and avoiding both transpositions.

 Right, 7.h3 is the modern main line. It's true that Black has no chance of winning that endgame but he should be able to hold it (easier said than done, but nothing that a few days with Fritz and his buddies won't solve). I can understand why it may turn some people off the MB, but personally I think if that's the best White can come up with, it can be regarded as a victory for Black...and there's plenty of respectable deviations available if Black really needs to try for a win. I believe theory has turned to Nxb5 as a better try for White anyway