Sicilian Defense Question

What is the main difference(s) between these two variations:

1. e4 c5 2. Nf3 d6

and

1. e4 c5 2. Nf3 Nc6

Basically, bringing the knight to c6 before playing d6. I know that Nc6 allows for variations such as the accelerated dragon. I ask b/c I notice that some players have a preference for one or the other. Thanks.

I THINK d6 is defensive and Nc6 attacks/controls the center.

Black sometimes in the Nc6 Sicilians intends to push d5. Also it rules out the Najdorf. It also delays dark bishop development. Maybe g6 fianchetto will happen. Also maroczy bind (c4 and e4 pawns super controlling d5) positions come up far more often in Nc6 Sicilians so if you don't want to play against that then d6 is the right way to go. The variations in d6 include the Najdorf, Dragon, and Scheveningen.

Nc6 Sicilians include Sveshnikov, Accelerated Dragon, and Kalashnikov (my personal least favourite of the 3) black gets a backwards d pawn that can be quite annoying to deal with.

Don't forget there are also other ways to continue besides d6 and Nc6 like e6. Going for the Kan or Taimanov. Often in the Kan you can use your light squared bishop more than you can in the French defence. I don't play the French for this reason but I do like the French Sicilian since black can make normal moves and be good for the most part.

@Masquerade #1
I believe the Nc6 is simply the old Sicilian line, old as in history, and in ECO code it seems to be called Old Sicilian as well.

See e.g. the famous "immortal" game Alexander McDonnell vs Louis Charles Mahe De La Bourdonnais from their 1834 match :
http://www.chessgames.com/perl/chessgame?gid=1001165

In that game the old fashioned way of playing with e7e5 and then Qf6 (or Qe7) is used. Later the Sveshnikov was invented as an improvement for black in that very e7e5 line. The old line (once played by Mikhail Tal by the way) is nowadays hardly played anymore. The Sveshnikov is relatively popular. I believe that GM Shirov still plays it, and even GM Carlsen (despite his positional grind down style) has done so.

Then at some point the Najdorf and the Dragon defense related to the d7d6 line became popular.

So, to summarize :
Black would play d7d6 to go for Najdorf defense, or some Dragon defense line, or Kalashnikov defense.

Black would play Nc6 to go for Sveshnikov, or the old lines with e.g. Richter-Rauzer for white, or the Lowenthal line (as in above mentioned match game), or some Dragon defense lines, or the Grivas (Qb6) lines.

As you can see the Dragon as choice overlaps a bit there, but still the reasons to play either d7d6 or Nc6 are completely different.

Correction, Kalashnikov should also be mentioned in the Nc6 part, but without Ng8-f6.

"the main difference"? Nc6 is committal, in that now the N can't be played to d7. The game will take different directions.