Which one is better? And in what ways?

I have used both in my car and I like the Z series better because of the 7 inch screen and the 3d landmarks...accentuates coolness. The D3 has a better look and quicker Ipod interface in my opinion. For my car, the double din 7 inch looks much better. The bluetooth capability is pretty much the same.

Also, the Z3 has a built in hard drive which you can store your CDs on it and don't have to worry about keeping them handy...

It pretty much boils down to whether or not the hard drive feature is worthy to you...