I wouldn't even consider the PPK. I've had too many bad experiences with the Interarms guns. The short butt Cobra can be fitted with Agent stocks making it as much a pocket gun as a J Frame Smith.
Give the Cobra a thorough look for frame cracks. I traded off a really nice nickel Cobra with the shrouded barrel a couple years ago. Big mistake. $500 is about $100 less than the going rate assuming it's a nice gun. There's a little confusion about the number of gens. in my mind. I'm thinking a 3rd gen Cobra would be a short butt with shrouded barrel while a comparable DS would be a gen 4? Some say that DS's actually have 5 gens, with a minor change around 1950.
NRA Lifer, Poor Speller