Both of those cars are definitely not your everyday sightings. You will have trouble finding parts for both of them. When it comes time to sell the Pontiac MAY be easier to sell because it is pre 48.
You have to look at what your budget and decide what you can do and what you will have to pay to have done. Hand fabbing panels to repair rust will cost you quite a bit of money. If you and your son want to buy some DVD's and do some research or take some classes you can learn to do that yourself and save a lot of money and do quite a bit of bonding along the way. Most teenagers are not willing to invest the time needed though. The Plymouth will not need the body work but will still require a lot of parts searching. You may want to find a Chevy or Ford as a first project together as they will be easier and quicker to build.
But if you do decide on either the Pontiac or Plymouth you could end up with a ride that is very unique like these. Good luck and let us know your decision.