Being an owner of all three types, here is my two cents.
Corns.
Fantastic snakes, especially for beginners. A 40 gallon would be like a palace to an adult and I'm sure they'd love it. Hundreds of morphs and styles to choose from. You can't go wrong with a Corn, IMHO.
Ball Pythons.
Another awesome choice. Easy to take care of and they
usually like to hang out. Tons of morphs are available, but can be pricey. Normals are just as fantastic, though, IMHO.
BCI Boas.
If it weren't for these guys' sizes, I think they would make great starter snakes. They are slow moving and patient (at least mine are!), but grow rather fast and rather HUGE in time. As mentioned above, this one will eventually outgrow your 40 gallon and you'll need to get a bigger enclosure.
So...my suggestion would actually be to get the Ball python or the Corn. Where you've owned the Corn before, you've got the experience and where you haven't owned the BP before, you'll get to have new exposure to a different animal.
