The further you sit from them, and/or the larger the room, I would get the larger bookshelves.
If your heads are very close to the surrounds, the RS42 may give you more diffuse audio, which may be desired.
Are those bipoles, or dipoles, or can they be switched to either mode?
Dipoles are designed so that you sit in the null. Both dipoles and bipoles will react more with the room and its boundaries/layout. Bipoles will give you greater coverage than the other designs.