Spelljammer for me. I love both, but Planescape seemed a bit limited. Factions controlling things in Sigil, gods controlling outer planes, and so forth. Spelljammer on the other hand let you be the master of your own destiny (and ship).
Spelljammer also allowed you to be very creative. Simply carve out a new crystal sphere, populate with planets and strange fantasy astrophysics, and you get an entirely new environment. Most of the planes (aside from pocket planes and some abyssal layers) were already defined in vague terms.
But I love Planescape too. The two are my first and second favorite D&D settings.