Cav - no doubts at all!!
I have an eight year old Black and Tan, and three, now teenage boys. He was ideal with the lads when younger.
He cuddles and sits on my lap (fantastic for period pain - did you know that Charles' mistress had terrible period pain and she was significant in persuading him to breed cavies?). He will also walk as far as I can cope with; he races around the woods for an hour a day if I let him. I think Cavies will actually adapt very well to the amount of walking you want to do.
And most importantly he toally adores me, I can do no worng. He is as loyal as...well a loyal thing!
BUT - YES I AM SHOUTING - BE VERY VERY INFORMED ABOUT CAVs BEFORE BUYING
Mine has the full quota of KCS related disease; syringomyelia, MVD, he has had both hind legs fixated due to dislocating knee caps...and more. He is npw eight ans we are facing some very tough decisions about brain surgery for him Incidentally he is now up to over twelve grand on vets fees....
Cavies are fab and some wonderful people are trying very hard (with it appears some success) to eradicate these bred in diseases but you need to be very very careful about the breeder you support by buying.
I can send you some very reliable and informative links if you like but start by having a good look around the Cav club sites. Do a google on SM in cavs...and find out how your chosen breeder scans for fitness prior to breeding. I would totally encourage you to spend a lot at a good breeder; they need support to carry on the good work!
One more thing; pedigrees and Kennel Club recommendations are worthless.
But...I would still choose to have a cav, mine is utterly irreplaceable. That photo of BoF when she was poorly just about sums up everything I love about cavs with a good dose of racing around the woods as a bonus.