Like Mike said, the 5200 is 80W because you can use both channels at the same time. The 500 won't let you do that. So you are still at the equivalent 40W per channel on the 5200. If you want 2 usable soldering irons in 1 package the 5200 is the way to go. If the Metcal is going to be your primary station and you have another station as a secondary, maybe the 500 is the way to go. I have a 500 and a Hakko FX-951. I got the Metcal after the Hakko for a really good price, just couldn't pass it up.
Don't worry about 40W output. The 500 has massive thermal capacity and recovery. It will handle anything you can throw at it. I added 5W bleeder resistors onto about 1/2' or so pads in 2 35A Astron power supplies with a 2.4 mm tip with almost immediate wetting of the solder on the filter capacitor leads.