Compare the conditions of the life for a French Serf and a Roman Slave?

Roman Slaves: Slavery was an institution deeply rooted in the ancient world, both in the Mediterranean and in the Near East, and not even Christianity when it emerged and triumphed as the state religion (in the fourth century) seriously challenged this institution. It does not follow that the bulk of the labour in the Roman economy was performed by slaves.
That may have been true of large parts of Itlay in the Republican period (under Augustus there were still 3 million slaves in a total Italian population of 7.5 million) but it was no longer true of the empire as a whole.
Slaves were an investment, and at least one Roman agricultural writer advised landowners against using them in contexts where too many might be required (for example, for harvests) or where their health could be damaged (for example, by malaria).
These considerations were not based on any sympathy for the slaves but on hard economic calculation. On the other hand, if the Roman upper classes were often brutal towards their slaves, ordinary people did sometimes show much more compassion.

French Serfs: Serfs cultivated plots of land, but these belonged to the lord. Much of the produce from this had to be given to the lord. They also had to work on the land which belonged exclusively to the lord. They received no wages and could not leave the estate without the lord’s permission.

The lord claimed a number of monopolies at the expense of his serfs. Serfs could use only their lord’s mill to grind their flour, his oven to bake their bread, and his winepresses to distil wine and beer.

The lord could decide whom a serf should marry, or might give his blessing to the serf’s choice but on payment of a fee.