Whimsicott because I had good fun with the one in Black2. Jumpluff is annoying and can go screw itself; at least it's easy to win against noobs with Whimsicott - there's no sleep powder to eat every other turn.
I have a soft spot for Jumpluff ever since it was introduced. I love its colour scheme and how it the fastest Grass type in the Sun. Whimsicott on the other hand, wasn't something I was that fond of at the start (is it a sheep/goat or a fairy? I didn't like how the backsprite looked as well). While I can start to appreciate the design more I like how Jumpluff just exemplifies a carefree nature, its blue and white body floating amongst clouds in a blue sky. One of the pokes that really defined my love for Johto designs and region, really always wanted to use one.
Take a look at my signature... have any idea which pokemon I prefer?
To be honest, both Pokemon aren't really nostalgic for me. Whimsicott can never be nostalgic since it's Gen 5, but I used it in game a lot and fell in love with it. I had a few Jumpluff cards, but I never used it in game. The annoyingness is the main theme of these two mons. If you've seen any of Shofu's videos, you'll know how annoying as heck Jumpluff is to handle. The same goes with Whimsicott, but I like Whimsicott better in battle. (It sweeped a team of 6 legendaries with Stun Spore/Leech Seed/Substitute/Hurricane. Admittedly, it was against an inferior opponent by a long shot). Honestly, though, the design is the main difference. Jumpluff honestly looks like an Oddish (my favourite Gen 1 grass type) with a few odd-lookin' balls around it. It's just such a horrible-looking Pokemon.
Whimsicott, on the other hand, has a nice balance of cute and cool. It's a frickin' spore ram! I mean, don't you just want to snuggle right up next to it... admittedly, the Gen 5 sprite doesn't do it justice, but that doesn't take anything away from the design.
I don't have any particular affinity for either pokemon, but seeing a friend of mine vs. Cynthia's Spiritomb ("It has not a single weakness!" - "Duuuuude") was one of the things that got me into pokemon.
Plus, I may have said it before but I FU@@ING HATE PRANKSTER. Grawr.
I'm not that young, I was around in the RBY era. It's just that I started playing pokemon recently for the lulz and really got into it. And Cynthia is... recent :P