whether to use a penalty one or a no-penalty one depends on the skill you need for your target creature. If your magery skill is 75, and you're fighting something that gives its best gains in the 50s, then you need to use the weapon with a penalty. If the critter gives best gains in the 80s, use the no-penalty one until you hit the 90s, then switch to the one with penalties.
BTW, If you want a Spell channeling (-22 I think) mage weapon butcher knife that's 100% poison damage (it's useless for golem training because of it having "Hit Harm 38%), just yell and I'll meet you in Luna.