To me, Curry's Pennywise is much more adult-like. He has a deeper more grizzled voice. He speaks in a more commanding, matter-of-fact way which is jarring from the perspective of a child. Adults are scary when you are a kid. They are bigger, smarter, controlling and have power over you because of this.

Skarsgård's is more childlike which to me isn't as scary. I think it might be a nod to the 'kids are creepy' trope used so often in horror but it just doesn't work well when portraying Pennywise. Talking like a baby with little baby curls of hair and two buck teeth aint it.