The battle between all the obesity drug makers will take a while to pan out and see which drug gains more market share. But overall, I think that Arena Pharmaceuticals will be the top seller against Qsymia, because of its safety profile.
For Orexigen it is still up in the air, because they won't go up against an FDA panel for approval until 2014. Also we don't know if the "Light Study" from Orexigen will be enough to show the FDA that Contrave is safe and effective. Qsymia has started earlier in sales for now, and it still remains to be seen how successful it will be in the long run.
Arena pharmaceuticals is set to bring out Belviq sometime in 2013, and it should sell well because of its safety profile. If I had to choose one winner out of all the obesity drugs, I would choose Arena's Belviq for long-term success.