While K is very pretty (And some watch it just for that reason), KHR started out as a gag manga, so not so much...
BUT!! Later on, the artist who works on KHR's manga does improve a lot, and so the anime gets a bit better looking.
Animation differences aside, K and KHR both have similar characters, themes, and if you've seen KHR first you might keep thinking to yourself, "Gosh! GORA must have seen Reborn!"
Well... It does seem that way, but it's kind of unlikely.
To spell it out simply for you, I'll make a little list.
KHR and K both revolve around Families/Factions/Gangs that sort of thing.
Both have color and attribute related abilities... I mean... HOMURA is just a bunch of Storm Flame users pretty much...
K has characters that seem similar to KHR's characters. Especially in the case of Mikoto Suoh... And if you like Xanxus, you probably will like Suoh.
It's very hard to go into further detail on the similarities of both series without spoiling either of them... But I hope you'll catch why I'm recommending KHR to K fans once you start into the later seasons of KHR.
KHR is a lot longer than K, but it's worth watching. Both series have made me laugh and cry, and I hope that if you look into KHR, you'll see why it's just as charming as K.