I'm yet to go into the Shivering Isles, and I read that if you go to Mania, you receive Dark Seducer light armor, and if you go to Dementia you get Golden Saints armor. Which one is personally better for those who have completed the main questline for the Sheogorath?

Its not which side you choose initially, its the side that you choose later on in the main quest

Also it is really a matter of which armor suits your character, light or heavy:
The full set of level 20+ dark seducer armor (including helmet) gives 35 armor rating for a weight of 34.5
The full set of level 20+ golden saint armor (including helmet) gives 52.5 armor rating, but for a weight of 138
However, if you are a master in both light and heavy armor, they will have THE EXACT SAME armor rating (52.5) and weight (0)

Golden Saint armor is heavy armor, Dark Seducer armor is light. They are of the best quality for their type and at lvl 20+ the Golden Saint armor is comparible with daedric, while Dark Seducer armor is comparible with glass. Which one is best depend on your skill with the armor-skills.

In my opinion: If you are a male character, then both armors look horrible. If you are a female character, the Saint's Armor looks better.... but..... the Saints Armor, if I recall correctly, takes up three clothing slots, which means you lose out on enchanting two other slots of armor for your character.

Both armors use the same equipment slots, so no difference there. If you're a Master of both Light and Heavy Armor, the defense is the same and encumbrance is zero, so it's all a question of what looks best on your character.