>>43973707XD improves on Colosseum on nearly every aspect. The only things Colosseum does better is having better (protagonist) designs, a somewhat more challenging story (even thogh XD is far harder in its postgame than Colosseum is) and utilizing a darker Pokemon setting while not feeling like some tryhard edgy fanfiction like literally every "Mature" Pokemon fangame.
Colosseum is really jank, and has a ridiculously low budget and it shows in pretty much the whole thing, even though I love Colosseum as a game. Unless you really adore using shitmons and snort Double Battles like cocaine, play XD.
