1989 astro van: starter or ignition switch?

i have a 1989 chevy astro van. it's been cranking slowly all day so i was planning to buy a new battery tomorrow.unfortunately, it wouldn't start when i came out of the grocery store this evening. lights and interior power is fine. no click, no crank. fuel pumps a whirrin' away. i played with it long enough, and it "slow-cranked" up. kinda low on dough right now and too low on time to make a mistake-starter, or ignition switch? why?