The answer is both.
Cloud-to-ground lightning comes from the sky down, but the part you see, comes from the ground up.
A typical cloud-to-ground flash lowers a path of negative electricity (that we cannot see) towards the ground in a series of steps.
Objects on the ground generally have a positive charge. Since opposites attract, an upward streamer is sent out from the object about to be struck (mostly the floor).
When these two paths meet, a return stroke zips back up to the sky. It is the return stroke that produces the visible flash, but it all happens so fast – in about one-millionth of a second – so the human eye doesn’t see the actual formation of the stroke.