Different release schedules
Japanese sets are usually released first, and English sets often combine or adapt content from Japanese releases. Set names and numbering can differ between the two.
Different packs and boxes
Japanese booster packs and boxes are typically smaller than English ones. Compare contents on the product page rather than comparing prices directly.
Who buys which
English products are playable in most international tournaments and appeal to players and collectors in English-speaking markets. Japanese products are popular with collectors who value their print quality and earlier releases.