I only noticed the difference is the character sprite. Is there any other difference between this 2 versions?

There are fewer enemies in some sections, enemies are placed slightly differently in others. Weapon placements are different in some stages as well as health drops (you can see this in the very first level, there are health drops and you can pick up the saucer weapon before the boss). Level layout and bosses are the same though.

I personally find JP an easier run of the game - there's less lag, more health, weapons are in slightly more useful places, etc.

I'm not sure if it's actually different or if its just my brain playing tricks on me, but the hitbox for the basic attack feels different too. In US its a frying pan swinging down, and in JP its your head dashing forward, and I do find that they have different hit ranges and arcs. That might not be accurate though and could be a perception problem on my part.