I wondered this for a time, but I think that it doesn't really matter which one you use. Tbh, I even put it to the test with OpenGL and OpenGL ES. Each and every time, the same result... it worked!
Now, when you have two or more different extensions (i.e. GL_NV_texture_rectangle vs GL_ARB_texture_rectangle vs GL_EXT_texture_rectangle), then chances are you have a vendor specific extension, where using the ARB version is optimal.