Wee Strom has more power.
ABS is the same from bike to bike. I believe both bikes use bosch units so there's no advantage from one to the other.
Wee doesn't get as good a fuel economy.
F650 is lighter. Holds its fuel down lower, but it's battery up higher.
Battery terminals are difficult to get tightened down correctly, and lots of plastic to remove to get to the battery.
Regrease steering head bearings immediately.
Some sort of reoccuring water pump issue that I don't now much about.
Torx fasteners everywhere..
From riding with people on the 650GS, I'd say it's build quality is lower than that of the Strom. Seems to not handle the vibration, and shaking of off road as well. Lots of plastic bits go flying.
Both bikes will cruise on the interstate comfortably.
Both have a top speed over 100 mph. Strom is faster by a few mph. 105 vs 110.
IMHO, you get more bike for the money with the Strom.
What ever you do, don't get the crap BMW bags if you get the 650 GS. Rack is crap, and the bags really don't seem to take a fall over well at all.