Part of your decision will depend on the range of tools you have access to. You can achieve a simple construction for your Z axis by putting the rails and the ball screw for the gantry mounted axis (Y in my case, X for many others) on the front face of the gantry. This can allow your Z axis back-plate to be a simple flat plate, but it does give you more overhang than other designs and exposes the ballscrew to shavings. This was my choice largely because it was the easiest to achieve in a limited workshop. Getting the ballscrew out of the way behind the gantry is much better for it but you'll need more than a hand hacksaw and a few files to be able to cut the pieces of the Z axis frame accurately enough. Unless, of course, you're a lot better with a hacksaw and files than me!

One of the comments I have seen on here a few times is not to use the smallest sizes of rails for the Z axis itself. It seems like a good idea for reducing overhang but you need enough room between the fixed and moving plates for the ballscrew anyway and it just makes life more complicated to have too narrow a space.

Kit