I've played both X360 and PS2 versions and honestly they're almost the same. Visuals are worse of course, but gameplay (which is all that matters) is exactly the same. Plus, the PS2 version comes with a WIRELESS Fender guitar. The X360 version comes with a corded guitar due to licensing issues with Microsoft's wireless "technology".
Really, the only downside is the lack of downloadable songs, but the lower cost (we bought it last week so we payed full price >

) offsets that. Also, a "track pack" with additional songs for the PS2 and Wii versions will be out soon.