When adding my stout facuct I called around to gas distributers in my area. I was suprised that some would only fill CO2 tanks, and some would only fill nitrogen tanks. So I did some research on the topic and here is what I found:
I looked at the specs for both tanks made by the same supplier "Luxfer". The CO2 tank specs were 1800psi service pressure regardless of capicity. The Nitrogen tank specs were between 2015 and 2216 psi service pressure. This means a CO2 tank filled with CO2 would be filled only slightly less than a nitrogen tank.
Both tanks are rated DOT-3AL. This DOT rating allows for CO2 and Nitrogen to be filled in these cylinders. Here is a link to all of the gasses that can be used: DOT-3AL Gasses
Finally Luxfer states right on the website for thier CO2 tanks: "Luxfer offers a broad range of aluminum cylinders for containment of CO2 (carbon dioxide) and related gas blends."
So after the tank research proved that it was safe and legal to fill a CO2 tank with Beer gas, I decided to research the regulators. The Taprite regulators I have all say do not use with nitrogen, and I couldn't find documentation on the subject, so I called them.
They stated that blended gasses were fine through the CO2 regulator since the pressure of the fill was only 1800 psi.
Both the Nitrogen and CO2 regulators output the same psi (0-60), and while the Nitrogen Regulator has a 3000psi high pressure guage, the CO2 regulators are rated for 3300psig.
I have found that the internet myths that it is illegal to fill CO2 tanks with Beer gas, and that Beer gas is at a much higher pressure than the CO2 tank can handle to be false. It really comes down to what tank your gas supplier will fill, and what your preference is.
For me, the redundancy of using the same tanks and regulators, as well as the cost savings on the tanks made the decision easy. Beer gas in CO2 all the way!
If anyone finds information in this post to be inaccurate, please let me know!