If you're not comfortable with the rotation and find yourself making mistakes, Mastery does come in on a very close second overall, and is actually preferred on some fights over Haste. Since prioritizing Haste speeds up the rotation dramatically, if you're not rock solid on it, I would suggest going Mastery and adding Haste as you feel more comfortable.
(As an example for the "which is better for DS" question: Ultraxion will favor Haste, but Gumball Boss favors Mastery (because you can bloody the boss up (as this is Mastery's main source of damage) just before swapping to bleed a bit on the adds.)
In short, it's whatever you're more comfortable with. I know plenty of people on this forum who have said they prefer the Mastery priority build because they like to plan their next moves and have the time to check buffs / debuffs / bleeds as well as the floor they're standing on for odd colored crap.
Personally, I prefer a Haste build because unlike those mentioned above, I *hate* waiting for energy. :P But even then, I have a round-about preference for Haste, not blindly following what Rawr tells me to do or the "reforge something on every piece to haste" build. (I generally use Rawr because wowreforge is set to Mastery and I'm lazy. :P) I don't take Rawr as an absolute, just as a starting point. If I upgrade a few pieces and feel my rotation is going too fast (either I'm energy capping or making mistakes) / slow (waiting for energy regen), I make adjustments based on that.