I have to say that after experiencing both Colosseum and XD, i have a stance towards the two games, which are both great games but with also great flaws that do not really ruin their credibility.
Colosseum:
Groundbreaking at the time of its release. It introduced a much darker and edgier atmosphere to the Pokemon universe, and it introduced Double Battles as the main way of playing the games, which of course was a much different ball game compared to the Single Battles that most people were used to. If there is one thing i would like to say about Colosseum is that it was the more challenging game between the two because every trainer battle felt that they were always of a higher level relative than what the player's current party really is. Hell, the final battle against Evice was something. Most players who do not grind and just casually playthrough the game as much as possible would always be TEN LEVELS LOWER than Evice's party.
Regarding the purification of Shadows, it was fun until it got a little too tedious. Some may argue that its a way to get "attached" to the pokemon you want to purify, but when there are so many things to do, it just gets frustrating.
XD:
It had some gameplay improvements and anti-frustration features that were fixed from Colosseum. It even did include wild pokemon that can now be caught, which adds to the lighter atmosphere of the game, as well as an additional means to get some useful members for your party. It also had some really fun distractions and mini-games to help the players get hooked.
However, despite the inclusion of wild pokemon through pokespots, the way Pokespots were executed was severely flawed. Basically the whole process is that you leave some bait and wait for the wild pokemon to get there. Sounds simple? yes except its easier on paper than on execution. You can be on the process of an important part of the game such as infiltrating the Cipher buildings but suddenly you get a notice on the pokespot rader. If you don't get there on time, you lose the bait and the pokemon that you could have caught. Basically, there's no way around it.
Also, unlike in Colosseum where every trainer battle felt harder because they were always of a higher level than the player (even by 1 to 3 levels, which is still a lot), in XD things got a little easier where it went back to the core-series roots were majority of the trainers are now weaker than the player relative to the time you face them. Some trainers who were seen in the last game even had their pokemon LEVEL DOWN (ehem Eagun and his pikachu) just to join up with the trend of making trainer battles easier for the player.
********************************************
Both are incredible games on their respective rights. If i wanted the more challenging game (including the management of some flawed features) then I would prefer Colosseum. If I wanted the lighter but arguably less frustrating game then XD would be my cup of tea. These are games that don't get the attention they deserve because THEY AREN"T CORE SERIES POKEYMANG GAMEZ CUZ THEY ARENT IN HANDHELDZ..