The chip size is 4:3 (or 3:4 whichever you like). To minimize space lost to stuff other than photos so to speak, the book should have that dimension ratio as well.
however when doing the double spread thing for landscape you will loose either on the sides or on the top depending on your cropping.
4:3 double page will be 4:6 so basically 2:3.
When you would do a book with dimensions 3:2 it will mean your portrait photos will have lost space.
You have more portrait images so the logical thing would be to go for 4:3 IMO.