You'd have to say it's a bit of both, really. As business partners, Scrooge and Marley are totally obsessed with making money all year round. They'd never let a little matter like Christmas get in the way of generating profits. To them, Christmas is an irritation, an inconvenience, a time of year when people seem to lose sight of what's really important in life: making money.
Therefore, it's not really surprising that Scrooge and Marley choose to work on Christmas Eve. Not only does it mean that they can squeeze every last penny out of their business but it also means that they don't have to deal with what they regard as annoying people, all full of the joys of Christmas.
Marley's greed receives its just reward in the afterlife: his troubled ghost is forced to wander around in shackles for all eternity. The same fate lies in store for Scrooge if he doesn't change his ways. Thankfully, Marley's ghost and the three Christmas spirits are on hand to help old Ebenezer save his soul.