As a lefty who has only before fired weapons with the military, I have to say that I would strongly recommend the P99. I guess I am a bit biased (the browning sucks) but using the browning for so long as a left handed shooter was very annoying. I know a lot of lefties just shoot with their right hand in the military to avoid the trouble. I just got my P99 from Canada ammo a few weeks ago, and I have to say I love it. I haven't been able to get down to the range with it yet, but the handling is superb with the ambi slide and mag release.
I do agree with the earlier comment that the decocking button is somewhat awkward to use as a lefty, but definitely manageable. Canada Ammo was selling them pretty cheap (somewhere in the $800s), hopefully they still have a few in so you can snag one, also these are the Walther direct models not the S&W, so you dont have to pay a price increase for the Walther version.