Hi! I am a proud dog mom to Gus, my 10 year old Cavalier. This might be long winded, but here goes...
About the breed groups...there are two different ones, the CKCS-USA...(Cavalier King Charles Society) which is not affiliated with the AKC, but it's been around longer....and the AKC affiliated one (I forget the name) --which at the time I got Gus 10 years ago was not as well thought of. The breeders I talked to at the time all belonged to the first group, but weren't all part of the AKC thing. (This was right around the time that Cavliers got popular, and when they were first recognized by the AKC, and competed at Westminster, etc....) At the time, some standards were different (like the CKCS group required more medical testing, stuff like that) I don't know if the situation is still the same or not, but it's something to consider.
about the breed itself.... One thing to be concerned about is the extremely high percentage of Cavaliers that wind up with Mitral Valve Disease...Some get it early, some late, but almost all of them have it towards the end of life. (And it can vary in severity too--Gus is lucky at his age to only have a low-grade murmur.) Other things are normal purebred concerns, like luxating patellas, hips etc......I don't want to scare you off, though. I love Gus and every cavalier I've met has been the same--happy, active, mellow---a great family dog that can run around like a loon, then collapse on the couch for a snuggle and a nap.
I'd insert a picture of Gus if I could, but I couldn't figure out how to do it. But he's a ruby (red), with a blaze of white down his chin (it's whiter now than ever--but made him Pet Quality not Show and thus cost less $$). Oh, and I almost forgot the best part--he's named for Gus Gus from Cinderella because he was the pudgiest puppy in the litter. I can't take credit for that one though because his breeder was a Disney fan--his Dad's name was Peter Pan and his mother was Princess

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Good luck with whatever dog you choose.