This is actually a pretty hard decision honestly ;;. If you had asked me the same question a year ago, I would of instantly said Whirlipede but now that I think hard enough about it, I actually can't decide which one I like better out of the two. I used to hate Garbodor's design, I thought it was trashy (no puns intended :]) and just awful, but it's starting to grow on me actually! I think it;s adorable,those eyes and I know some people will say 'Oh it's uncreative and stupid!" but really, then again each generation has its flaws in design and creativity, so that's what I have to say.
For Whirlipede, personally I'm also rather fond of it, I recently started using Venipede and actually I must say it's a strong one! :D. I highly underestimated it and I was wrong for doing so ;;, also it's design is pretty cool. Although competitively I don't think it would be too good unless it held an Eviolite or something, even then it's still pretty bad. :(. Although in-game its wonderful! :3.
Aaand as for Garbodor competitively well, it's actually brilliant! I'm disagreeing with you Hikari, honestly Garbodor is great. It can set up Toxic Spikes,Spikes and lots more, also it learns some neat poison moves. Overall I think my vote goes to Garbodor this time!