Given a choice, I'd grab the Spirit Blade. It offers 4 AD against undead and the maximum defense option with a shield, probably 5 DD, but maybe 6 DD. The Dragon Spear offers the same 4 AD against dragons and the same 5 or 6 DD. Yes, the Dragon Spear can offer the advantage of diagonal attack, but 4 AD is very situational to a few specific Quests. Many Quests offer multiple rooms of undead, so that's lots of opportunities for 4 AD to be effective. The diagonal attack is still available for the owner of the Spirit Blade by purchasing a Longsword, if that is what he wants.
No big reveal here, but my

on what I feel counts.