NPD, the product sales tracker in the US, has revealed that Guitar Hero: World Tour outsold its rival music game, Rock Band 2, by a factor of 2 to 1 in 2008.

World Tour, releasing simultaneously across four platforms in 2008 (Wii, PS2, PS3 and Xbox 360) racked up 3.4 million units in sales, while Rock Band 2, on a staggered release schedule from the middle of September to the middle of December, has sold 1.7 million units.

While these numbers are significant and comprehensive, one must take into account the staggered release of Rock Band 2, as the last two platforms on which it was release were the PS2 and Wii – the two consoles with the largest combined installed base.