I don't think we really need to compare Bee Dee and K. Rool. They're both great for different reasons.
Dee is great because he's a character who started as a bit part in a Superstar minigame, received a promotion to the King's right hand man the remake, and then single handedly inspired a new ability for Kirby just so he could be promoted to full PC. Really, the Spear ability has become his trademark, just as much as Hammer is Dedede's and Sword is Meta-Knight's. Add that to his continued roles in Rainbow Curse and Triple Deluxe, and you can see that he's filled a niche in the Kirby Canon that's not likely to leave any time soon. It's a classic underdog story, and he's really earned a shot at a spot on the roster, especially considering that he offers some great moveset potential that's yet to be tapped. (Dropping the ball, Fire Emblem!) Even more so, considering that he also has the potential to use other weapons and tools that his fellow Waddle Dees have used, like Parasols and vehicles.
K. Rool is a completely different kettle of fish, being a main villain with a huge role from the get-go. He actually underwent a bit of a reverse story to Dee, starting out strong and relevant, then slowly being relegated to bit parts as DK lost his luster, and not being brought on board when the revival happened. Of course, considering his runaway support, and the fact that he does have a strong history with the Kongs, all he needs is one big break to come back into his own as the rightful ruler of Kongo Bongo true villain of the DK series!
(Sorry the Dee paragraph is longer, but I felt I didn't need to tell the K. Rool thread why he's so great.)