Personally I just cast soul trap as a spell instead of having it on weapons, then put some extra damage boost into the weapon enchantment instead since I can land soul trap on the enemy before it even gets into melee range most times and have it last the duration of the fight. Having it bound to a hotkey makes it useful for swapping in. Also means my weapon enchantment isn't wasted on creatures you can't (or don't need) to soul trap.
But that's just my preference.
But that's just my preference.