Where are you going to ride? In the woods, you will appreciate the 200, a bike that can haul you but you can haul too, if necessary. For desert, you will appreciate the power of the 500. If both, Rick Siemen says the 200 is all the trail bike anybody needs.
I weigh as much as you do, and I have had some fine days on an XR200. I mostly ride a DRZ400, which has more than enough power for me.