Since at no point in TPM is there even a hint of what work Shmi does as a slave, I don't see how any argument could be made about what droid is more appropriate.
Although if they had been switched, I could hear this argument being made by RLM.
"What good is a spaceship mechanic droid to a slave? Wouldn't R2 make much more sense belonging to Padme, who actually owns spaceships?"