Very likely, there is no difference between the two of them. One way to test this is to select "Tango" and take several screen shots of the icons. Then select "Automatic (Tango)" and do the same thing. Then compare the screen shots.
I have version 4.0.0.1 for 32 bit and 64 bit Debian. (I use Ubuntu.) There are some differences from what have been mentioned: Automatic is associated with Oxygen as in "Automatic (Oxygen)". Galaxy is still the Default selection. And yes, there is a separate "Oxygen" choice which contains the same icons for the Standard toolbar as "Automatic (Oxygen)". I checked.
The 4.0.0.1 version for the Mac has "Automatic (Tango)", "Galaxy (Default)", and "Tango" (plus a few others). So there are some differences in what icon sets are available by OS. I don't have access to the RPM version.
--Dan