The PSP version of Echochrome costs twice as much as the PS3 version, but Siliconera explains there is reason for the extra money. All of the answers are found over at Impress Watch (Japanese).
The PlayStation 3 version is actually "Echochrome: Jokyoku" or "Echochrome: Prelude" in English. For 1,800 yen ($17) you get a get a sample of Echochrome by tasting 50 puzzles. The 3,980 yen ($37) PSP version contains 100 puzzles and multiple modes including a variation where you have to avoid walking dark echoes.
Fortunately, both versions come with canvas mode, which lets players create and upload puzzles on the PlayStation Network or transfer data via the PSP's Ad-Hoc mode. With this new information the PSP version is more tantalizing. However, the cheaper PlayStation 3 version supports users-created content, which devalues the addition of the extra 50 puzzles.