At 20 weeks I doubt the pup would've picked up that many bad habits. And if it has, it can be easily fixed. I got my pup at exactly 5 months old--her previous owners had neglected to crate train or potty train her properly. It was a mess--she would do her business everywhere, and hated being in the crate. After a couple of weeks' worth of training though, she did really well. She hasn't had a potty accident in about 2 weeks now--pretty great for a 6 month old!
I would choose the 20 week old too, especially if I've got a full time job or young kids. 13 week olds are much harder to train because they have very short attention spans and their bladder is about the size of a pea (LOL), and you cannot take your eyes off the pup for even a second.
I got my first dog at 11 weeks old, and it was a LOT of work! If I had a lot of time on my hands I would pick the younger puppy, but with other commitments, an older puppy would be good. You really aren't missing out on much--just photo ops but then again you'll be so busy, you probably won't have that much time to take that many pictures anyway.
Cavaliers act like puppies for a long time anyway! I've known 1 year olds who still act like 5 month olds.
Cadence the tricolour Papillon (DOB: July 2nd, 2009) & Lyra the blenheim Cavalier (DOB: April 28th, 2011)