In Dark Souls they just scale so they have more HP deal more dmg etc. Of course Dark Souls best part is the PvP which MH doesn't have. But for PvE MH is king compared to DS.
I only not agree with this part. Per example, you meet around 10 Titanite Demons in a Dark Souls playthrough, these have the same graphics and holding the same weapon. Well, 90% of them have a different moveset and most have a unique move.
I agree that the MHU offers more in terms of bosses but you make it sound as if DS just recycles through graphics for monsters, and while it does for some of them, they still variation even amongst the same enemy unit.
the addiction for me comes from the learning curve. A casual gamer will never get past the first 25% of the game (without being rushed by friends). It's hailed to be one of the hardest games ever and yet people are now pushing to complete it in the one hour mark. There's so many paths to complete the game, so many builds. And then there's the PVP which is what has me still playing :S