When your opponent plays the King's Indian Defense, do you go for 3. Nc3 or Nf3? Or another move altogether? What are the positional advantages of each? I personally go 3. Nc3 because I love the four pawn attack... It leads to fun games (for me). But I don't know what's actually positionally better.

i assume you're referring to after 1.d4 Nf6 2.c4 g6? Obviously, if you like to play the Saemisch variation, you need to avoid Nf3. In general, it seems as if you're always going to want to play Nc3 while you nmay want to defer or avoid Nf3

This one's pretty easy to answer. 1.d4 Nf6 2.c4 g6 3.Nc3 is far better than 3.Nf3 for most players. 3.Nc3 allows white to choose between the Saemisch with f3, classical lines with Nf3, and the sharp Four Pawn Attack with f4.

The drawbacks of 3.Nc3 are almost non-existent. If white plays 3.Nf3, he only limits himself, not his opponent.

I want to be absolutely clear: I don't believe 3.Nf3 is actually an objective mistake since White can transpose into mainlines fairly easily. But there is almost no good reason to play 3.Nf3 over 3.Nc3.

The only line where 3.Nf3 is preferred over 3.Nc3 is the fianchetto variation. And even there, play almost always transposes. If you really want to play the fianchetto lines, you're probably best off playing 3.g3 first! That's Avrukh's recommendation, based on what he plays against the Grunfeld rather than the KID.

yeah if you do Nf3 your best choice is the classical line, but Nf3 is much more flexible. If you are going to play the classical eventually, they are the same.

There is one major consideration for playing 3.Nf3 over 3.Nc3: 1.d4 Nf6 2.c4 g6 3.Nf3 d5 4.cxd5 Nxd5 allows white to play 5.e4 without allowing an exchange on c3. This discourages Black from playing the Grunfeld. But Black scores well from that line too.

3.Nf3 does encourage Black to play 3...c5 though. The point is that Modern Benoni lines where white has already played Nf3 are considered to be favorable to Black compared to lines where White can still play f2-f4.

1.d4 Nf6 2.c4 g6 3.Nf3 does discourage the immediate ..d5, but Black will play 3...Bg7 which he always does, and now the move is White's: does he play 4.Nc3 where 4..d5 is a Grunfeld, or does he play something else?

Avoiding the Grunfeld is not a reason for 3.Nf3.  If you want to avoid the Grunfeld at this point you need something like 3.g3 or 3.f3.

In general when your pawn is on c4 the N really wants to be behind it on c3.  Just about the only Queen pawn opening where White shouldn't deploy the N there ASAP is against the Slav.  1.d4 d5 2.c4 c6 and now 3.Nf3 is regarded as the most careful move order.  3.Nc3 gives Black a couple of additional options, 3...dxc4 and 3...e5!?, the latter of which is unplayable against 3.Nf3 and the former playable but much weaker.

Hope this helps.

True, GCB. I was trying to find a rationale for 3.Nf3. Perhaps the offbeat 1.d4 Nf6 2.c4 g6 3.Nf3 Bg7 4.b4!? would be the best reason for that particular move order. But then Black simply plays a KID and white needs to justify that extra pawn push.

I don't think that's anything to be afraid of though. Clearly, 3.Nc3 is the preferable move order for most people.

Thanks to everyone for the very helpful discussion. I'm gonna stick with 3. Nc3. I need more practice (as white) against the Grunfeld and Benoni defenses... as far as I can tell, I've only played two games against each defense, and I don't know them at all. But that's another story...

I like 3.f3 and 3.Nc3, as well as 3.g3, but not 3.Nf3

Nc3 is typical... Nf3 often transposes, but blocks the 4 Pawns' Attack and the Samisch Variation.