I believe the Maxflow 3000 system can do both...
I think it has an anti-syphon in the tank, and you can buy an adaptor to fill it with CO2. Check with smartparts though.
You also get an annoying backward thread on the bottle to really help you drive everyone nuts.
And yeah - under NO CIRCUMSTANCES put air in a CO2 tank. If you are lucky you will blow a burst disk, if you are unlucky you will have a DIY fragmentation grenade. You might be able to get away with putting CO2 in a dive tank, although if you have one of those you'd be better off using it for air.
BTW, are there any HSE requirements for handling/storing bulk CO2?