Honestly, you could go either way.
The Hellbents have a ton of float and K2 skis are always a bit longer than measured, so I think you could manage with the 179 as well. It depends on what you want to do.
I'd say if you want to ski slowly, ski tight trees, jib around, do spins and maybe even take them to the park for shits n giggles, go 179. If you want to ski faster, bigger lines, big drops and want a lot of stability, 189 for sure.