So currently Im in te SI getting some dope for the duke. And I'm interested in what land of the SI you like.
My opinion:
Landscape: Mania. Just look at dem colors!
NPCs: Dementia. I'd rather be threatened than talking to a paranoid person. Especially that homeless woodelf xD. And the NPCs of dementia are just way more interesting.
Enemies: Dementia. The Mania enemies are like fantasy jungle creatures. Not my style.
Most prefered one: Mania. The landscapes are just amazing. Sometimes even better than the heartlands.

Isn't it called felldew? Personally I like both environments equally, and don't see either one as "better" or "worse". Unique? Certainly.

I was always a Dementia man personally.

I could never make my mind up, sometimes i was one and other times the other.

It depends on which character I take there. On my most recent playthrough, Lord Kain is more Mania aligned.

I like both but where is that land of in-between? That place of stability called SANITY.

No such place exists in the Shivering Isles.

Sheogorath stands for madness. Sanity and insanity is for the weak.