There are many series, typically shōnen titles (Bleach, Naruto, One Piece, Fairy Tail, etc.) that have both the original manga version and an anime adaptation running at the same time. Because the anime events are behind the manga events, you're faced with a tough decision; do you read the manga first and optionally watch the anime as well, or do you ignore the latest manga chapters, and watch the episodes, even though it's further behind in the story? Vote and let us know why in the comments.
Note: To be clear, this isn't a simple "Manga vs. Anime" poll, it's specifically about long-running franchises with both a manga and an anime version available.