Mixing
Both formats come out of the same session, with the same decisions behind them. So the spatial version is the record, not an interpretation of it.
Atmos goes wrong when it's treated as a format conversion. Someone gets the stems, hits the spec, ships it, and the spatial version is a different record than the one you made.
I work the other way. Whether I mixed the stereo or you're sending me someone else's, the job is the same: the Atmos version has to be the record, with the same intent behind every decision. Producers send me records they mixed elsewhere for exactly that reason.
Taking both formats together is also the cheaper path by a distance. Bought separately they're $1,300 a track. Together they're $800.
