Those are two VERY different torts to chose from. I think I would do box turtle or russian...
The two you chose, one hibernates, one does not, one gets to be 100 pounds the other not...
Curious...what made you choose those 2 species?
How old is your son?
I agree that a Box turtle or a Russian Tortoise are good choices if this is his first turtle/tortoise.
Sulcatas require an enormous amount of food and space and a sturdy enclosure. If you have cold winters, you will need an indoor enclosure too. Adults have been known to walk thru walls. My 20lb Sulcata already has enormous poops!! When it needs a vet, who is going to pack it around when it weighs over 100lb?
Good luck
Patsy