I would choose a very humid basement coz at least my car is under the shade. I would just think of it as parking under a tree in the midday sun. Unlike all day under the sun, it will subject my car's paint, interior to extreme beating. Then to make matters worse, it will suddenly rain then gets sunny again.
liquid spills sa leather seats are easy to clean but it a b**tch kung nakababad sa init ng araw.
anyways i'd always prefer parking under a shade. kahit pa humid.
parehong hassle at mainit, but i would choose to park in a humid basement rather than under the sun. at least sa basement walang UV rays to damage the paint and interior.