a chaplain would be a much better addition to the assault squad... leave the book-worm to join the termies as they can get away without the chapy's litanies of hate.
assault marines are acctually one the worst assault units in the game simply because they're still just space marines... any other dedicated assault unit will usually wipe the floor with assault marines, so a chaplain with his re-rolls is much better than the book-worm's pointy death stick! honestly, those re-rolls make all the difference in how effetive the squad becomes and allows the squad to assault larger squads. the litanies also help to mitigate the shooting casulties you'll like suffer on the way in too
cheers!