Mister Furley seemed like he was more "a part of the gang", he went along with the others and joined in on their escapades. Plus, his over-the-top exaggerated mannerisms fit perfectly with the show's physical-comedy "style".
Roper was just a cranky old pervert.
And you don't have to have a good memory, just insomnia. It's still on every night at like 3am on Nick-At-Nite.