Well, I like the original Pikmin much more than the sequel. It has a better atmosphere, and is more balanced game overall. Exploring the tranquil distant planet is a great experience that should not be missed, I say.
The sequel, by comparison, is more or less a dungeon crawler with ridiculous bosses and a fucking impossible '0 Pikmin dead' goal. The monotony kept me from beating the game 100% (didn't lose any Pikmin though, and I did beat the final boss, which admittedly was pretty awesome). Still, treasure hunting in randomly generated dungeons = not for everyone.
Oh, and you probably should play the original first, if only to see how the levels have changed in the sequel. The first level is... Different, to say the least.