Hey,
Well, I think you will have your hands full with either bike in the tights. The XR650 probably weighs more, but the WR is no light-weight. I think the smooth, torquey and "lazy" power delivery of the XR's is king in the woods, just my opinion. The WR probably puts out a bit more power than the XR650, plus it should rev higher. For MX and the like, I would think you would be better off with the WR because of more rigid forks and better suspension overall.
Everybody in these forums will tell you to run the other way from the XR and run and get a YZ/WR or KTM, but you already ruled out the KTM. If you want to MX then the XR probably isn't for you. However, for any other reason, including tons of USABLE power, and bullet-proof reliability the XR will do you right every time. Good luck, and remember, pick what bike YOU want, and what YOU think will be best for where YOU ride.