I'm not sure if I would describe MS Finance programs as necessarily less selective than MBA programs but they do emphasize different things. As you mention, age and work experience tend to be more important for MBA admissions than for MS Fin admissions. However, MS Fin programs often have higher prerequisites in mathematics than one encounters at MBA programs.
High performance in a demanding MS Finance program is helpful in showing that you can succeed at in a demanding MBA program. To the extent that a business school is more familiar with the offerings of its own university's own MS Finance program, this should help you as well. A few schools, such as Vanderbilt, even let you apply the courses from your MS toward an MBA at that school.
Princeton:
"While the program does not require formal work experience as a requirement for admission, prior work experience can be a definite plus. At a minimum, we expect applicants to have completed one or more internships (for instance over the summer while in college)."