Quick Macro question

It depends on the subject.
If you have the 70-300 and EX-25, use it and figure it out. Then think about the 35 mm, which will give good magnification, but will require you to be closer to the subject.
I would say, tiny flowers - 35mm, because you can take your time and get close.
Bees or bugs, 70-300, becuase the you don't need to be 6 centimeters from the subject.
If you can try a 35mm with or without the ex-25, it will help you understand when one lens would be preferable over the other.