That does seem hard to tell, what with that white band around the waist. I'd make a one-piece jacket as well, but only put the zipper (or whatever closure) to the waist, and then cover it with the white belt that ties in the back or on the side.

With things like this I generally figure out what works best as I go. If you have the money/materials/time (which most cosplayers don't, I find) then I would make a rough example of each style in an inexpensive cotton and then try them on and see which looks best. Though having said that, I think your best bet would be a long coat.

Sorry for the long and ramble-y reply, I really can't shut up when is comes to sewing and cosplay, but I hope it'll help. Good luck!