You can switch specialization but it costs gold, of course. How much I am not sure but the countless BS guides on the net can provide that information to you if you care to search for it. They will also tell you that if you switch, you will loose the ability to craft any "specializtion" designated recipes.
You might also lose the ability to employ any "specialization" designated items you crafted. I may be wrong on this point -- can someone verify or correct me?
As to what you choose, I can only speak from pre-BC experience: you choose according to your specialization. If you are a DPS warrior, you probably want to be a weaponsmith of your preferred weapon. If you are a TANK, make your own armor. While BC might provide some specialization specific recipes -- and it would be stupid if it doesn't -- everything good you can make from about 270 - 350 and which you learn from a trainer is BoP so you cannot make it for any one but yourself. (Don't get me started on how retarded that is.) In Outlands you will be able to get recipes from about 300 - 330 which are not BoP (Fel Iron armor and weapons available to all smiths) but everything you are looking at in a list form pre-BC trainers is just for you.
As a side note, don't bother to grind reputation pre-BC just for faction recipes. Outland greens have made things like the Timbermaw recipes, once allegedly sought-after blues, crap. The Timbermaw boots, which require six Arcanite bars among other things, completely sub-standard for lvl 59 chain. Not sure about the rest of the faction recipes but do your homework before wasting your time.