What are the advantages/disadvantages of the different OpenGL options for Android? Any significant performance difference between the two? Does OpenGL 2 reduce the number of target devices by much/any?
Advantages with GL2.0 = support for shaders (among other things) = more advanced rendering techniques.
Disadvantages with 2.0 = easy to kill performance with too complex shaders 
Not really; IIRC most, if not all, Android devices supported by Unity Android also has support for GLES 2.0.