both. depends on what i'm doing both have their place. starch when i want it stiff, sizing when it just needs a little more body, but i don't need the extra ...
i do not use starch on Batik's, they press beautifully with water (i also never put water in my iron, i keep a spray bottle of water on the end of the ironing board, with the starch and sizing)
but if i'm pressing, cutting a looser weave (like homespuns) i will usually need starch.
i use sizing on Silks, other delicate fabrics, ones i don't want a starch to build up on.
it's just a matter of preference, and what your encountering at the time.