Along the lines of what DarkLink said, for this to have true comparison value, you also need to be listing the benefits of a Gargantuan Creature over a Monstrous Creature, including -
1) Poisoned Weapons only wounding on a 6+
2) Instant Death weapons having a reduced effect
3) Stomp
4) Etc.
I have not listed out all of the pros/cons, but for this to be a valid discussion, all data needs to be presented.
Edit: I do note that you list these things within one of the paragraphs, but not as visibly as the data that supports your side of the original argument.