Can I ask why you guys use soul trap for multiple seconds? I always use just one, it works perfectly as the weapon I use always delivers the killing blow anyway and it gives me more charges.
I enchant for the longest duration possible with Soul Trap on a bow, the reason being that at lower levels it's doubtful that my first arrow is gonna deal the killing blow. If I have a follower or allies in the fight (by choice or quest-related) they are going to be attacking the same target(s) and it may very well be one of their weapons that gets the kill and it may be within seconds of when my arrow hit but before I can get another one into the same target to "reset" the trap. If the trap lasts longer I feel there's a better chance of getting the soul since as long as it's active it doesn't matter who or what actually kills the target.
Seems to work well for some multiple attacker situations, for instance wolf packs and other lower level enemies, especially with a ranged magic follower. Sometimes I can get a hit with the bow on 2-3 different targets and then between my follower and myself they all get finished off while a trap is still active on each.
With a melee weapon I enchant for a 1 second trap since most likely I will be striking continually until the target is dead and resetting the trap over and over anyway, so I'm not worried about the trap lasting long enough to allow for kills that come from somewhere else. And it does get you more uses per charge.