Cause of Shuckpluff:
Which one has a better design?
Which one would win in a fight?
Which one is more useful?
I love both of these Pokemon, but I prefer Shuckle. The design really appeals to me, and the defensive stats are gigantic. It's a decent wall, especially in Sandstorm. Jumpluff has things it can do too, however, such as high speed and support moves such as Sleep Powder, Rage Powder (for doubles/triples), Leech Seed, Stun Spore, and more.
As for who would win in a fight, I dunno. Jumpluff don't generally carry Toxic, I don't think, but Leech Seed would be difficult for Shuckle to handle, especially since it can't Rest it away. But the rare Power Trick Shuckle could probably destroy Jumpluff. It really depends on the moveset, I guess.