Sprite transparancy VS full color

Hello :)

I have a question, which is the best option. Sorry for my bad english i am from the netherlands, hope you understand what i try to ask.

For example:

- I have a sprite 16x16 with a rock in the middle, the rock is surrounded with grass. So all of the pixels in the 16x16 sprites are filled with color.

OR

- I have the same sprite with the rock, but now surrounding with transparancy. This way i can place the sprite on a grass layer..

Which option is the best for memory usage etc?

Thanks for your time!

Tim

Memory use is identical.