They are all good options, I think for that it would be more of a matter of personal taste and budget. The way a lot of people size their enclosures is to see that the measure of the front and one side is at least equal to the total length of the snake. That being said, you don't want to go too large because it will most likely intimidate a new snake in a new environment. If you want to get am enclosure that your snake can grow into, get one that will suit the adult size of the snake American then when the snake is young, fill it up with lots of clutter (fake plants, extra hides, rocks, crumpled up newspaper, etc.) To take up space in the enclosure and make it seem smaller. Then as the snake grows you can gradually remove the stuff you had in there.
For the tank with the divider: you don't need to feed the snake in a separate enclosure. Feeding in the enclosure does not cause cage aggression, that is mostly a myth (i say mostly because some snakes will be cage aggressive on their own, though that is rare with ball pythons).
Whichever one you choose will be a good choice for the snake, and I'm glad you're doing your homework and learning this all before bringing home baby.
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