Presumably trivial damage, but NV will be handy on any fight with recurring burst every 90-180 seconds. HotW in contrast is preferable on any fight with one major high damage phase, such as Sha of Pride or Galakras (though NV may also be handy on the latter, depending on which phase poses the biggest problem).
There's going to be a big difference between 10 man and 25 man, yes. Bigger raid means far more potential targets to place the mushroom under, giving you more precision/flexibility. I all comes down to personal preference either way. Everyone should try both and see which they prefer.