Between 39 & 49, I would suggest 49 for flying serpent kick and tier 2 PVP talent. But I am inclined to agree with Tzvetan when he recommended you take up 59. WW really should get countered by any other melee spec in the game but they are capable of being extremely strong and even overpowered in some situations.
I just leveled a monk 1-80 over these past few days (leveled strictly through PVP mind you) and from what I've noticed as WW is that they're a lot stronger then given credit for, but the only reason they are even remotely good is the "Control the mist" PVP talent which makes them somewhat of a livelord. I think the spec has a ton of potential at 59
Neither, TBH. WW does well enough at 39, but you're missing all the skills that make the class fun to play (chiefly flying serpent kick, leg sweep, and ToK). At 49, you have a few more abilities and a lot more mobility, but the damage isn't great and the class essentially pales in comparison to every other melee. There's almost no reason to take a WW to a BG when any other class just does the job better. Sadly, it's kind of a lose-lose situation. If you're really set on playing WW in a lower bracket, I think you'd get the most out of 59s (though admittedly, that's a bracket I don't play in).