That got me wondering too.  So I went to the USDA database and compared 100
g of zucchini and 100 g of crookhead/straightneck summer squash(they don't
list a yellow squash but I think this is close).

Zucchini has 16 kcal, 3.35 g of carbs, 1.21 g of protein, 0.18 g of fat, 17
mg of vitamin C, 200 IU of vitamin A and 120 mcg of beta carotene.  The
crookhead/straightneck summer squash has 19 kcal, 4.04 g of carbs, 0.94 g of
protein, 0.24 of fat, 8.4 mg of vitamin C, 150 IU of vitamin A and 90 mcg of
beta carotene.

The differences are small but IMO the zucchini comes out the winner.

Tonia