SS XC Freewheel vs cassette = Reliability?

I think a good quality freewheel should outlast a freehub conversion. Less access points for dirt and stuff to get in and do damage.

I believe freehubs suceeded freewheels primarily because the former allow the main wheel axle bearings to be placed on the outside, whereas with a freewheel they have to be inside. This limits freewheel hubs in terms of the number of gears that can be crammed on before the bearings are too far inboard to maintain a strong, rigid build. But if you have a wheel/hub built to take a singlespeed freewheel, this can be still be built pretty stiff as the bearings don't have to be pushed so far inboard to make way for multiple gears.

Use what you have. Freewheels are pretty cheap even good ones are.