In MHO the book by Davies is a very good repertoire book,
advocating some unusual sidelines. Rowson's book is somewhat older, but it has much more information on typical plans, pawn-structures, middle-game problems and even end-games.
If youre an experienced grunfeld player Davies is a good choice, otherwise Rowson is a better choice anyway.

I would need to know what side of the board you take, black or white...

I just went through the chapter and must say Rowson's is much better (of course Rowson's is by far best book on the grunfeld...)

As it happens I played as Kasparov in the line given (Timman-Kasparov) this summer in Benasque and a weaker player crashed me! It seems you have to play a la Svidler in  the 7.Qa4 line, although, Kasparov's moves where of course very natural... To be fair to Davies he mentions "Svidler's 8...Bd7 is possibly blacks best" but he ends there. If you don't know the line, what follows is no evident AT ALL.

Davies is more concerned about giving alternatives to established lines. Some have been succesful enough to become main lines, and some chapters where more successfully written than others.

If you are with the black pieces, get Rowson's! with white... Dearings is excellent, but you need a fantastic memory, so you may as well try any line if you know what you are doing!

Nigel Davies book is 100 x better than the Aagaard effort. (Even as a starting out book!)

Aagaard coverage is just VERY inadequate for the current theory level of the Grunfeld. Davies tackles that problem by recommending alternative lines where theory is too heavy.

Russian system particularly well covered in Davies' work

Nigel Davies' Grunfeld book has a reasonable summary of Russian theory. The latest book is  Aagaard's Starting Out Grunfeld, which made a favourable impression on me after some glances at the bookstore. I think this is one of his better books. As always he seems to be more frank and personable than most authors. There's slightly more theory here than others in the Starting Out series.

But of course, chesspub is best if you want the very latest.