As Avrukh says, 1. d4 players are divided into two camps: those who don't like to face tactical defences (mainly the Grunfeld), and those who don't like to face solid defences (mainly the Nimzo/Slav). So you can't go wrong - either way you are making the best choice for that particular approach to the opening.

I think there are three issues to consider which all have an effect on your practical results:
(i) which opening fits you best stylistically,
(ii) which opening gives you most pleasure to study,
(iii) which opening gives you most confidence over the board.

(i) is not only a subjective question but also an objective question. You may "like" an opening, but whether an opening objectively fits you however is difficult to answer without lots of trial and error. Playing many games and analyzing them afterwards, as well as hiring a coach will help.

(ii) is for me the most important but often underrated factor. If you enjoy studying particular positions you will certainly become better quickly at pattern recognition and understanding typical themes in the opening.

(iii) is probably related to (i) and (ii) but I would add that confidence is important in its own right. Refuting the opponent's opening play often requires a belief that the opponent has done something "wrong". Sometimes a few losses in a particular opening can dent that confidence, but if you truly have confidence in an opening you will continue to perservere with it and get better at it. For instance, for some years I had absolute confidence that White was better in the 4. Bd3 f5 English defence that I kept trying to refute that opening and in the process, learnt much about it. 

If I were you I would prefer the Grunfeld, simply because if you take pleasure in Grunfeld positions and studying Grunfeld theory you will become better at it.

Hi, I'm playing the Grunfeld again after a few years away from it, but it was one of the first defences to 1d4 that I 'learnt' when I started playing about 15 years ago.  I read Rowson's book on it and was hooked, even though it was largely way too advanced for me. 
My question is really about the difference between the Grunfeld and the Nimzo/ Queens Indian complex, as I am planning to use one of these choices as my main answer to 1d4 now, rather than the Dutch, QGD or Slav options I've punted over the last ten years or so.

I'd like to return fully to the Grunfeld as I like the position types, but despite this I'm forever reading or being told that it's too advanced for players below 1500, for example. 
I'm around that level or below, and I often play online in casual blitz games, against players who often don't seem to know more than myself in theory terms.
I realise that I should worry less about openings and more about tactics and other areas of my game, but I still like to talk about opening choices.
So, whilst I like the Grunfeld a lot, I have also experimented with the Nimzo and QID occasionally, and my head tells me that these are more solid long term choices than the Grunfeld, but my heart pulls me towards the Grunfeld.
Does anyone have an opinion on the theory that's required to become competent at either of the defences, relative to each other?
At my level I know this isn't key, but please discuss in general terms, at whatever level you wish, as the comparisons are what I'm interested in hearing, from those who've spent real time with either.
I would also have to consider the Catalan with the NID/QID complex, too, so I'm wondering if the theory isn't just as great, or greater, than a Grunfeld rep.  And I would be using something like the Safest Grunfeld, rather than Avrukh....
Any thoughts welcome !