All right. For teams with Charizard X, think Miami. They've got stars that demand your attention and typically make or break the team. They take all the credit too (yes that was a jab at Lebron). With Y teams, think more about the Spurs (in their prime. They're still good, but man do I miss the old days). Very team oriented. They have players that can step up and make a huge difference, but they rarely depend on all star performances as the team works together to win the game. You take your pick out of those, but don't let last season's fluke sway your oppinion.
Obviously I'm a little biased (Born and raised a Spurs fan), but that example does a decent job of showing how both have systems that work. X is the better solo pokemon, similar to how Lebron is better than just about everybody, but the Spurs still showed that they were their equal by the team having synergy, similar to how Y can support his team mates more. Hope that helps
While I agree Y has a great role as a supporter, I think it's potential as a sweeper is severely understated. It has the strong coverage, the power, and the boosting moves to pull it off once it's main threats, Heatran and Tyranitar, are cleared off the field (of if you are running Focus Blast, once the Dragons are cleared).
I actually use X as a wallbreaker on my team, and Y as a sweeper on a different team. I feel like X's strong mixed attacking stats combined with Tough Claws allow it to fill that role more effectively.