i don't think the goldfish will really mind what kind of gravel you use.
so it's about personal preference here.
sand is different to gravel, crap doesn't sink down into it, it just sits on top of the sand so you need to vacuum the surface of the sand regularly so it doesn't look bad.
with gravel, especially larger type, crap can sink down into it so you can't see it, which is good. then when you vacuum the tank you can just stir up the gravel to suck up the crap.
i go to mitre ten and buy a sack of river pebbles. under $10 to do the whole tank...`