Lost Worlds is better. I own both. I was excited for both and was disappointed in both, but I was way more disappointed in Colors. Such an overrated game, like the whole turd that is the Adventure series. Seriously some of the worst games I've ever played.
But yeah, Lost World was the first 3D Sonic game I've played that was mediocre. Well that's not fair. Colors wasn't bad. Just significantly worse than Lost World. Lost World was fun-ish.