Hi Trudy,
B. darwinii generally has quite stiff, small (1-3cm), dark green leaves, compared to the softer, lighter coloured, and larger (up to 7cm) leaves of B. glaucocarpa. The most obvious difference can be seen in winter, as B. glaucocarpa drops its leaves while B. darwinii doesn't. Also B. darwinii has serrations only near the tips of the leaves, while B. glaucocarpa, has them all round the leaf margins.
Check out the descriptive pages for these species on the flora section of this website.
Cheers,
Rowan