With proper set-up (humidity-holding substrate & minimal air exchange) either way should be fine. Personally I'd opt for the small one now, because later on it will be handy as a temp. holding area or useful for the next snake. I've subdivided enclosures before too, & that's a certain amount of work so it's secure & safe, & then you have to remove it. I'd just get a larger enclosure in the future.
That's what I'm going to do. Small one now and upgrade later. Thanks.