If you haven't bought any yet I'll second both waiting and Mechromancer, assuming you've looked at play videos and stuff and think it'd be something you'd like. In a sense you're going to get the most bang for the buck out of Mechro because it's a new class for you to play through the story again, along with any other DLC you get down the road. Of course if your playstyle isn't conductive to multiple playthroughs then it wont be as good...
If you have to get something now I'd say flip a coin. Both Pirate and Torgue have pros and cons to them. Pirate is a little more story driven and longer imo, but Torgue is just plain ole fun and more of what makes Borderlands fun (imo)... the randomness and badass things.
Mechromancer is quirky and has a skill tree that basically completely flips the playstyle I used with Axton on it's head...Pushing me to completely change weapon preferences, play styles, ect..
And the Pirate DLC was actually quite fun. Great characters, decent story progression, some neat new weapons and areas, good music, ect.. And the Skid (New vehicle) is basically the best thing ever.
Torgue DLC on the other hand....ehhh...It severely underwhelmed me. Wasn't digging most of the story events, the levels didn't do a whole lot for me, new enemies weren't so impressive...ect.. Played it with a friend I played through the main game and first DLC with, and we were both just wanting it to end the whole time. It just wasn't very impressive at all.