Per the game manual, there is a difference in characters.

JJ/Kato is easy mode - the character runs a bit slower, jumps a bit higher, doesnt slide/skid as much, and overall the level timers seem to be in line with the speed at which they run.

Jeff/Ken is hard mode - the character runs faster, jumps lower and not as far, and slides/skids further than JJ/Kato. Due to this increase in speed, this character always seems slightly ahead of the level timers, thus is in a bit more danger of falling/thrown objects.

Overall, playing as Jeff/Ken seems to be roughly 30 seconds faster than JJ/Kato.

At this time the only playable difference between the US and JP versions is the fart/spray mechanics. In the JP version the character fart, thus have to turn around, to direct the attack forward. In the US version this was changed to spray cans, and a forward attack. At this time this is the only known differences in mechanics.