Grunfeld or Nimzo Indian?

I've been playing chess for about 6 months now and have an OTB rating of 1460. I aim to get somewhere around 1800-2000 hopefully in a reasonable time frame. So I've lately tried to get together a full repertoire which I can play and improve with over time. The repertoire so far is:

As white:
1.e4 and go for scotch game against 1. ... e5, main lines of all the sicilians (I don't think it matters exactly which variations), panov-botvinnik against caro-kann, Nc3 french, main line scandinavian, austrian attack against pirc.

As black:
1. ... e5 as black going into main lines of whichever opening my opponent goes for (except I'm currently trying a classical defence against the spanish with 4. Bc5) and now we come to the d4 defences...

I've previously answered 1.d4 with primarily a nimzo indian and incase d4, c4, Nf3 I go for a queen's indian. Against trompowsky, london, torre, colle, reti I haven't learned that many lines, but generally develop as logically I can going into structures similar to the queen's indian.

But I've been starting to find the nimzo and queen's indian somewhat dull and it is the part of my repertoire I am least happy with. So I've been considering the Grunfeld instead and possibly picking up "The Safest Grunfeld" by Alexander Delchev and Evgenij Agrest. I'm a bit concerned because I guess that I should in that case not only swap my defence to d4 to the grunfeld, but also adapt similar systems against the english, colle etc and go for a fianchetto setup which feels quite different from what I am used to.

So my questions is basically: with regards to my current rating, my rating goal and my otherwise complete repertoire, does the Grunfeld or the Nimzo/Queen's Indian seem like the better fit? Maybe it doesn't matter and I should just play what I feel like...but at the same time I believe the Grunfeld to be such a complex opening I don't want to waste too much time into learning it only to find out it doesn't fit well for me. Perhaps the nimzo and queen's indian might become more interesting if I invest more time into understanding them better?

Any and all help would be appreciated.

Neither: KID FTW!! (for the win)

I used to play the Grunfeld but found it too mathematical (not necessarily tactical).  I like the Nimzo-Indian but usually just go for a Dutch against 1.d4.  Also, white could also fianchetto kingside so you might not even get the classical vs. hypermodern bout with its contrasting strategies you're looking for. 

What does that mean?

I've ordered "The Safest Grunfeld" now and hope that book will give me decent insight on the opening. I've also ordered a book on the Black Knight's Tango opening and might consider looking at that. But I generally feel the Grunfeld might be what I want in the long run, so I might aswell invest some time into learning it as best I can now, and hope it is at least equally hard for my opponents to find a good reply.

Given your rating and goals I wouldn't recommend wasting your time studying openings. If you want to reach 1800-2000 in a reasonable time frame as you say, you are much better off working on tactics, endgames, basic strategic principles, common middlegames from the openings you play, etc. There are some opening books that treat this topic reasonably well (Tzermiadianos's book on playing against the French comes to mind) but for the most part they focus on the opening and assume you know how to play chess afterwards.

I would say that if you want to climb up the rating ladder, I would consider learning a few more aggressive openings. I have played many openings against d4, and the QID almost always gets me a draw. If I want to win the game but don't want to know that much theory, I would probably go for a nimzo-indian and avoid more of the theory heavy opening like the KID and the Grunfeld  ( even though I have found great success with them). Against e4 I have always played the Sicilian, but the Caro kann is also super solid if your looking for a less aggressive game. For your white repertoire I would consider learning the Spanish and Italian games (especially the Evans gambit for lower rated players). 

Dude, op hasn't been online since 2016...