I've made a spreadsheet to figure out which is better. It's assuming 4 shots (on a combi-bolter for Hydra's Teeth). I'm now leaning toward's Viper's Bite for SMs/Custodes and Hydra's Teeth for less well-armoured opponents, as Hydra's Teeth doesn't have any AP on a combi-bolter.
I would've preferred to show it in a binomial distribution pie chart rather than these inaccurate, disgusting decimals though. You're not getting the distribution at all, but it's quick and dirty.
Edited by ChazSexington, 28 January 2020 - 03:05 PM.