Well, in a text font, such as you'd use for a book, you typically use the two-story variants of both letters. This helps people distinguish between a, q, and g. In italics, the a normally goes to one story, but the g more often than not keeps both stories. (But there are some text fonts with a one-story g.)

In any other kind of font, though, there aren't really rules. I'd say (non-italic) fonts with a two-story g probably almost always have a two-story a, but the reverse isn't true—tons of sans serif fonts have a two-story a and a one-story g.

Since single story versions have fewer horizontal strokes to stack up, they are often attractive alternatives for fonts with relatively thick horizontals, such as monoline or low-contrast sans faces. (Especially true for /g/.) Note that Gill Sans Ultra (to name one example) switches over to the single-story form.

thanks for the answers guys. I knew it was simple enough.

cheers

I've heard it said that the lc italic /g/ used when setting mathematical equations should be single-story, but have not yet heard a convincing rationale for why this should be true. I've seen it done both ways, and it doesn't seem to make a difference in legibility to my eyes. If anything, making /q/ and /g/ distinct is a very good reason to use a double-story /g/. I've seen one book that skimped on the typography and blindly adhered to the single-story rule and ended up using a very sans-looking single-story lc italic /g/ with Times italic to very poor effect.