If Whitney had released the same amount of albums as Madonna do you think she could have sold the same - if not more - records than her? Whitney released 6 studio albums while Madonna has released 12. (They've also released a similar amount of soundtrack/compilation albums).
Whitney has sold approximately 200 million records WW.
While Madonna has sold 300 million WW.
So if Whitney had released 12 albums could she have sold the same as or more than Madonna?
Yes without a doubt. She only released 3 proper studio albums during her 10 year peak. If she would have released more often Whitney would STILL be the best selling female artist of all time as she once was.
Whitney has bigger albums than Madonna.
Whitney had bigger single.
Whitney has bigger movies.
Whitney had more awards.
Whitney was more respected.
Whitney was a huge touring act.