Oh dear Gawd, are we going to go into the generalized rafters vs kayakers crap?
Seriously, like Rivermamma says, it depends on the kayakers, BUT it also depends on the rafters as to who can be good to have for safety. It also depends on the trip leader's leadership skills, on how the roles and expectations are set out, and what the attitude is among the different members of the group.
Do you have long-time kayaker friends that you can invite along and you know are solid and whose gear you're always happy to rig onto your raft? Or are you just going to go random internet pick up to staff your trip? And do you and your other rafter buddies share the opinion that kayakers don't serve any other purpose but to haul their soggy butts out after a flip? If this is the case you may have group dynamics that'll be a lot more challenging than the rapids.
Good luck getting your trip together,
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse