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my secondary army is tau... i was thinking about getting commander farsight however stoped when i realised what kind of rules this would strech my army 2 and in the end made me wonder which tactic is better Mont'ka (killing blow) or Kauyon (patient hunter) ?

This would be an entirely personal choice. As long as they are played right, I would think both have an equal chance of being successful. (Note: I am not a Tau player myself)

Do you know what the styles are (I am still trying to pick for my army)

Both styles can and should be utilized by any Tau army, and certain unit choices will reflect which one your army prefers. No matter your army list, if you apply proper tactics then you should be using both of them to defeat your opponent.
Mont'ka- The Killing Blow
Bringing your force to bear to demolish your target.
This would be like moving both of your Hammerheads into position to fire upon enemy armour/infantry. Mont'ka is straight forward and basically ends up being "destroy the enemy, move on to the next enemy".

Kauyon- The Patient Hunter
Luring your opponent to a position where they will be at a disadvantage.
If you look up different strategy and tactic articles/threads on dakka, as well as general tactic articles from real generals and such, even from as basic a game as chess, you can see that forcing y our opponent to make a choice, where either choice puts you at the advantage, is good for you. Also if you can set up a trap for your opponent, that too would be a good example of Kauyon.
If in an objective game, you were to leave an objective out in the open, your opponent may jump the gun and attempt to claim it too soon, that would leave them in the clear for your heavy weapons, however your skimmer devilfish loaded with firewarriors could easily capture such an objective on the final turn.

I'll take the patient hunter.

I feel like both can be used effectively, but i'm not going to throw my troops out into the open in order to get shots on the army. With the current state of the codex and rule book, most armies can field more units, or more powerful units than Tau have. I want to make the enemy commit and pick him apart piece by piece, and not give him the same opportunity.

personally, I have just started playing tau because I like ranged combat, but I want to play differently than my guard army. Tau are much more precise, and their units are can be much more well rounded (even if their codex isn't).