Here's a good video review of the CB500X -
http://www.motogeo.com/honda-cb500x-review/
I have not ridden the CB500X, but it has an advantage at highway speeds. But the RX3 has a lower seat, better ground clearance, more suspension travel, larger 18" front wheel (or optional 19"), spoked wheels and 70 mpg.
The RX3 is also 50 lbs lighter, fully equipped ... 100 lbs lighter if you remove the luggage and bars. Aftermarket accessories and parts for the RX3 will generally be cheaper than the Honda too. And CSC's service is consistently exceptional.
The RX3 has a few shortcomings that are easily addressed with inexpensive parts like sprockets, fork oil and a 55W headlight.
Overall, if someone were to give me a CB500X or an RX3 for free, I would still choose the RX3. It better matches the riding I want to do.