It depends on the surface you're parking on, (How level, how firm, etc.) but usually the centerstand is more stable.
SOme people will argue that the contact area of the bike on the sidestand is wider, and is more stable, but I take into effect factors like kids climbing on the bike, and people attempting to move the bike for some reason.
I don't think most people who don't ride bikes will attempt to move a bike on a centerstand, but might try if it's on a sidestand.