well for me i like them both heres my main reason,
toobing: i love toobing in wierd places or distances like from one map to the other and getting legit kills makes the boring mw2 interesting...
as for ninja knife this is fun when you get behind the whole team and knife them from behind and they wont know kinda relates to as if your stealth and no-one knows where you are...so they camp and shit themselves hahahah
but if i had to choose 1 it would be toobing cause ppl toob on HC SnD....which is just plain noob