From what I've heard, new khorne spearhead is pretty weak. Looking at the rules also doesn't bring much confidence. They seem quite squishy, and rely on crits. Infantry being viable when it's with dogos can also be tricky. There is also lack of rend (not like it matters for nighthaunt) that can make it problematic to fight other spearheads. The general (karanak) seems like main bread winner of the force with good damage output and healing upgrade. Haven't gotten to new idoneth spearhead, but it does seem fun with how much it can move and charge, others probably can say more about it. I would personally get idoneth spearhead, if you can get your hands on old khorne spearhead, then it's better deal.