My room is 22 x 19, with the TV/front soundstage on one of the 22ft walls. My system is mainly movies/TV, rather than music. I am going with front heights (or Presence, I guess, because I have a Yamaha AVR) because I read a bunch of threads suggesting that you probably get more information from front heights than rear surrounds - at least as far as HT goes.
Not sure if the answer would be the same for music.
Boston Acoustics Classic series L/C/R + front presence; Fluance XLBP Bipole surrounds; 2 x PSA XS15se
Yamaha RX-A3040; 3 x Emotiva XPA-100; Emotiva BAS-X A500