Ribbit King is already a sequel, actually. The first game was only released in Japan on the PS1, called "Kero Kero King".
Ribbit King, by nature of being a sequel and providing the developers with the opportunity to polish their presentation and game play, is the superior way to play Frolf, overall. I think it's quite interesting, though, to see the origins and the more rough concept and style they were trying to present through the PS1 limitations.
I would say check it out if you really like the game play loop, because it's just a little more simplified version of the same found in Ribbit King.