I like them both equally.
The Biker outfit fits the more modern feel of the Ware series very well and it highlights just how independant Wario is from Mario at this point, by dropping a big junk of the simmilaritys between their designs.
The Overalls on the other hand, are a classic and you don't mess with a classic.
What warelander said, though I thought Wicked Wario and Artsy Wario were also pretty cool.
I also enjoyed Wario's Mario Party 2 costumes, especially Wizard Wario and Pirate Wario (the first Captain Wario... wait, I'm sure the game had a special title for each costume. Maybe it was corsair or something? I remember the Western Land ones being named Deputies).