Panel de Pon is the original game released in Japan. Tetris Attack is the modified game released in Europe/USA with the graphics changed to Yoshi. The Japanese version has more "cutesy" anime-style artwork and Tetris Attack features... well... Yoshi. I'm not sure if there's a difference in the gameplay so I guess it's just a matter of personal preference.
Unofficial English translation is available for Panel de Pon (the Japanese release):
www.romhacking.net
yaworg.googlepages.com/translationprojectThey're the gameplay wise the same. Yoshi version was re-released later in Japan as well :)