Elixirs.
Transmutation mastery costs 4 primal mights. You then have to transmute primal might for about 15-20 days to break even on average. Because of the 24 hour cooldown, this mastery just isn't worth it. Yes, you could get lucky and get all of the primals back on your first transmute...but you probably won't. Maybe if you have one of the extremely profitable transmutes, like Primal Water to Fire or Primal Shadow to Water, this might be a good choice. Getting extra primals is really only profitable. It's not a useful ability for helping yourself do anything other than make gold.
Potions are easy to make and useful. More importantly, there is no cooldown, so you can mix up a whole bunch of potions and watch the ability proc a few times in one sitting. Problem is that there is no high cost potion. Look at how much Super Mana and Super Heal pots go for on your server. Is getting an extra pot everyonce in awhile really profitable? On the other hand, if you are just looking to get an alchemy spec to help your character, having extra pots can be nice, especially if you drink them often.
Elixirs is the best of both worlds. No cooldown, so you mix up a ton of major agility, major strength, major defense, and onslaught elixirs and proc quite a bit. Flasks also fall under the "elixir" catagory. Outland flasks can sell for as much as primal mights on some servers, so if your ability procs on a flask, you can rake in the gold.
To recap:
Transmute Mastery: not really helpful, but fairly profitable...after recouping the inital 400g expense.
Potion Mastery: not really profitable, but fairly helpful.
Elixir Mastery: Very helpful and very profitable...if you know how to make flasks.
P.S. If you are in a raid guild, I would definetly go with elixirs. There will be enough alchemists and enough time for all the crafters to get the primal mights they need. Potions are easy enough to make in large quantities. But buff potions will be needed after every death and extra flasks are ALWAYS useful considering the amount of mats to make them.