I'm currently trying to choose between Gnome shell and Unity 2D/3D for my Ubuntu installation on my laptop (I don't really like the dull look of some environments like xfce). Of the three, which uses the most resources (battery)? The least? I know that Unity will use more than its 2D counterpart, but I'm not sure how the both of them compare to Gnome shell.
The 3D requirements of Unity (and Gnome 3, I would think), such as compositing, effects, etc. are minimal and cannot be compared to things like 3D gaming or 3D modeling.
If you are using a integrated graphics adapter that supports Unity 3D, such are the Intel HD series, there will be negligible effect on your battery life if you use it. This is because the integrated GPU always idles at a minimum clock speed, and that clock speed is only raised (consuming a non-negligible amount of power) if it is loaded beyond a certain degree -- I seriously doubt Unity/Gnome effects would do that.
If you are using a discrete adapter with hybrid/switchable graphics, then more battery life is going to be used regardless of your DE unless you use something like bumblebee or vgaswitcheroo.
This reasoning may not be true in Ubuntu 12.10 and later, if the rumored 3D software rendering for old graphics cards is introduced. In that scenario, the 3D DE may actually use more battery life because the CPU is performing everything in software.