The simpler one, gdmlogin, provides just the login window you would expect, featuring customized logo and background colour/image, system menu, and face browser.

The second one, gdmgreeter, additionally supports themes. Selecting a theme in GDM's configuration GUI automatically throws in gdmgreeter.

But for me, trying to achieve the quickest boot ever, the main difference is... 5 seconds! So much is gdmgreeter slower than gdmlogin. And it's more CPU-intensive, too.

I don't know if the extra time must be accounted to the theme or it's just that gdmgreeter itself is heavier than its companion, regardless of the selected theme. The point is: if you don't have a good reason to want a themed login screen (and I don't see any), don't select any theme in GDM: your boot will be faster!