Ok some quick pics. Not a really fair comparison as the cameras used were different. The SS endo was taken with a DSLR while the HCG was taken with a point and shoot sony. But its the best i can do since i sold the SS one a couple of months back.
On to the pics:
The two bust are quite different. First main difference is the finishing. Somehow, the chrome plating on the HCG seems brighter than the SS one I had. Don't get me wrong, the finish on the SS bust is awesome, just different - somewhat darker in its finish, if that makes any sense.
Looking at the side you can also see that the SS bust is somewhat angled as if the skull was looking downward while the HCG one seems to be more neutral and looking forward. The curvature is also different with the SS skull looking more menacing while the HCG seemingly more human in its appearance.
Details on both however are top notch in my humble opinion.
Lastly the name plate on the two are different. SS chose to paint it on a flat nameplate while HCG chose the 3D one. The advantage of the 3D one to me is you have less to worry about if you decide you want to polish it (cause its less likely you'll take off crucial paint and leave the bust missing alphabets!)
The SS bust has a nice fade out effect to it while the HCG has some cool sounds that accompany it. I remember my friend coming over and me turning on the eyes and my friends going, "Is that it?" Only thing that might bother some is the HCG one turns itself off after 10 seconds or so.
That's fine cos i normally do not leave it on for more than 10 secs anyway.
The base for the HCG is also wider and seems more well balanced making handling the bust easier compared to the SS one. Again this is my experience and others may feel differently.
Lastly, the SS bust has the bonus CPU chip which the HCG does not. It's definitely a bonus and its a pity the HCG did not come with the same feature. However, considering the HCG bust is older than the SS one, its acceptable.
On the whole the HCG one edges out the SS one for me simply because its easier for me to picture arnold's face shape behind the HCG bust. The SS bust is sleeker and arnold in my memory did not have such sleek features.