So we have a party of both a paladin and a warpriest, bot worship Iomedae, whose weapon of choice is the longsword. I know about the longsword you get in book 1, so my question is, which character class would benefit from the longsword more?
It grows into a holy avenger - which is pretty much THE weapon for paladins. I am not sure that warpriests would get the entirety of the benefit from the item.
So, I would say paladin. There are some other nice longswords in the modules.