MM dungeons were harder, in my opinion.
OoT was my first Zelda game, and even then, it was easier than
MM.
MM's dungeons were less linear, more confusing, and had harder puzzles and bosses. Basically,
MM had a buch of Water Temples in disguise. Great Bay... Eh, let my brother do it, then beat the boss after dying five times. Man, I stank at
MM...
OoT just came natural to me. I liked
OoT's dungeons, and
MM's were boring, hard, and annoying. I hated the Swamp dungeon's eerie background music, too... *shudders*
But, you had better help in
MM on the dungeons, because Tatl was much more useful than Navi... But did we need help in
OoT? No...