Rick,
Obviously a lot of similarity between the Saratoga and the Seneca since the Seneca is just a “twin Saratoga”. However, they are two very different planes even with a lot of shared DNA. Obviously from the choice of the two I assume you want a comfortable plane that can carry up to six. So assuming that is true you need to decide on the number of engines. If your son is going to fly the plane I would suggest the Saratoga just simply because he will need a single to fly for now. Also, even if he got his multi right away (possible) the insurance would be horrible (or not available). So just for those reasons I would suggest staying single engine for now. Maybe save the Seneca for later! When he has got his commercial and instrument out of the way and has some time built up it will make more sense to go multi.
Also, make sure you are aware of the Lance and Cherokee 6 as these are earlier versions of Saratoga and may be a better choice from a cost vs benefit standpoint. Having said that the Saratoga is a great plane!
Let us know what you come up with.
Eric




