If it is possible to make this argument at all, we must recognize that it is only possible if we are talking about physical and material matters. We should never argue that it was emotionally or psychologically better to be a slave than to be a poor white person.
Physically and materially, it could arguably be better to be a slave because slaves had security of a sort. Poor whites had no security. There was no welfare system and no social security in those days. If the members of a poor white family could not work, they had no way of supporting themselves. This would be a problem, for example, for a man who got injured and whose children were young. They would be unable to work and his wife would need to care for them. Meanwhile, they would have no source of income. Elderly people had to hope they had children who were able to support them once they were too old to work.
Slaves, by contrast, did not have to worry about these things. A slave who was injured would still be fed until they got better. A slave who was too old to work in the fields would still be fed. They might have to do whatever non-strenuous jobs were available, but they would be fed. A slave would not starve to death or freeze to death for lack of a home and clothing.
Of course, this argument can touch only on physical things. It is unlikely that having someone own you and your spouse and your children could ever be better emotionally than being free.