I'd use a hub for a folder, as it's more space efficient. The small motors are nowhere near as heavy as the large ones, because they're a casing containing a pair of dril motors, whereas the large hubs are actually one big motor with the windings around a cast iron wheel.

Depending on the design of the bike, a thin rack battery might be best, as it can stay on the bike when folded. Some 10ah 36v batteries are really compact now, and that's a lot on a folder.