Christina, while she has vocal talent, has spent all her career singing like she's the voice of the century when she's not even close, and just ends up trying to do more than what she is capable of, whereas Janet's vocal talent is a lot more limited, but she sings within the limits of what she can do instead of what she thinks she can do.
So yeah, Christina can do more with her voice, but Janet is effective in the little she does with hers. She doesn't strain, screech, push, force, yelp, grunt, or make her voice grind like it's going through a meat skewer. Quality, not quantity.
This is priceless.
Keep them educated.