They're both unique. I never really played them back then but gave both a try recently. I feel like DK1 has aged better, especially with DK FX. It feels like it could be a modern indie game. It actually looks gorgeous, imo. DK2's move to fully 3D makes it look very dated and the way the UI/UX has changed also makes it feel more dated to play than DK1. I found it to be more frustrating just in how you interact with the game.

I really enjoyed both games though, so I absolutely recommend playing both.