Many feel that Jan was inferior to Marcia, but I disagree.  Look at the similarities.

Jan was busted by Bobby for not setting the table
Marcia was busted by Alice for not turning off the hi-fi.

Jan thought Aunt Jenny was ugly
Marcia thought Charlie was ugly.

Jan was devastated over wearing glasses.
Marcia was devastated over wearing braces.

Jan created an imaginary romance with George Glass.
Marcia created an imaginary romance with Dr. Vogel.

Jan was named the most popular girl in her class in Jr. High.
Marcia was named co-hostess of the Jr. High Banquet.

Jan wanted to be an only child.
Marcia wanted to disown Cindy after she lost her diary.

Jan helped new girl Kerry Hathaway.
Marcia helped shy Molly Webber.

Jan volunteered Mike to build sets for school play about Benedict Arnold.
Marcia volunteered Carol to perform a duet for the Family Frolics.

Jan was allergic to Tiger's flea powder.
Marcia had new school-itis.

Jan wore a silly brunette wig to Lucy Winter's birthday party.
Marcia wore a silly brunette wig to trick Kerry Hathaway.

Jan tried to rub off her freckles because she was ashamed of her appearance.
Marcia tried to hide her nose in the water fountain because she was ashamed of her appearance.

Jan got a swollen head after being named Most Popular Girl.
Marcia got a swollen head after being named Juliet in the school play.

Jan failed to make the pom pom team.
Marcia failed to make head cheerleader (Greg chose Pat.)

Jan promised Herman that Greg would help him with algebra.
Marcia promised her class that Davy Jones would perform at the prom.

Jan frantically tries tap dancing, being a drum majorette, and acting trying to find herself.
Marcia frantically joins yoga, karate, scuba diving, and the Westdale Boosters trying to find herself.

Jan is tricked by Myron the mouse.
Marcia is tricked by Jerry Rogers.

Jan froze during school debate.
Marcia froze during driving test.

In a historic moment in Brady history, in episode # 104 "Marcia Gets Creamed,"
Jan was chosen over Marcia by Mr. Haskell to work at the ice cream parlor.