You almost always want the offensive weapon, since 30% Elemental Attack is just too much to pass up on. The skill-buffing is better for most HP weapons, excluding Herc and Morgan. Also, Shingen's weapons fulfill a different role:

1. The offensive weapon allows Shingen to use PF without outside help
2. The defensive weapons lets your bursts hit like a truck, but Shingen will need help from the party

Most of the time, the offensive weapon is better even for Shingen since, again, the 30% Elemental Attack is just too much. It's something to consider if you own Belial yourself and are running a friend Belial as well, though.