Assuming you really do mean a 300TD (the wagon) and not a 300D (with turbo) I own one of each...
Well, Click and Clack of Car talk say there is no reason to teach a modern-day teen how to drive a standard transmission - they are going to be extinct within our lifetime.
That said, with the minimal info you gave, I would opt for the 240D IF the seller has records documenting the mileage. (Same can be said for the 300D). The odos on these cars are wildly inaccurate, and I never believe milage figures unless they are verified somehow.
The 4-speed in the 240 is a bit wanky compared with other transmission. Gotta baby it, especially from 1-2 gear. No power shifting like in a ricer - that was a lesson I had to learn and I came to MB from a Nissan pickup!!!!
The wagon is VERY practical. For better or worse, your son could get 6-7 of his buddies in there
It is incredibly comfortable on the highway - we use ours as the family car. My wife putts around town in it and loves it for that, and I drive us to visit relatives or on family trips and I love it for that. The amount of cargo space in there is amazing!
the price seems reasonable for each if they are as you describe with minimal rust and working AC.
Good luck. I love both of 'em. I think you can't go wrong with your background and skills!
*EDIT*
I wouldn't hold your breath for the 30mpg on the 300TD.