I have seen some that are 25 and others that are 35. You are referring to Olympic sized, correct?
They are both good for different things. You may find that the curl bar is shaped better for your wrist angle and therefore will allow you to lift heavier weights with less pain.
Tests have shown that amount of resistance is greater on the bicep when curling with a straight bar as opposed to a curl bar. The tests were done using the same excact weight.