On March 19th two versions of Echochrome materialize in Japan. A physical UMD disc for the PSP costs 3,980 yen ($37) while the downloadable PlayStation 3 edition only costs 1800 yen ($17). Sony hasn’t announced any differences between the versions other than portability and if that is the sole difference I’ll stick to the more reasonable PlayStation Network download.
Take note that these prices are only for the Japanese release of Echochrome. Occasionally, Sony Computer Entertainment America alters prices for North America and when they do the resulting game is often cheaper. Piyotama, Go! Sports Ski, Jeanne D’Arc and Patapon are a few examples that come to mind.
Update: Thanks to Walaber's tip we know the reason why.