Here are some quick comparison shots. The first one shows the Gentle Giant on the left, then the Kotobukiya R2 & C-3PO, and finally the Hasbro. It's hard to tell in the pictures, but the color of the Hasbro isn't nearly as nice as the other two. It's quite pale in comparison. The other two are a nice deep gold, and the GG statue is actually gold plated.
The second shot shows the the Gentle Giant statue with a Sideshow figure. It is smaller than I had remembered. It has a tall base, so it would work in a pinch, and it looks less toy-like than the Hasbro (which I think is too big). Still, it's clearly too small. The Kotobukiya is actually the nicest one overall, but it's too darned short to display with 1/6 scale figures. Another inch or so and it would be very close to perfect (although it only has articulation in the head).
I guess my conclusion is that the Hasbro is still the best stand-in at the moment (especially considering the price difference), although the other ones are much nicer when taken as stand-alone pieces.
Anyway, here's a photo to illustrate the point I was making about the difficulty of making an accurate articulated C-3PO. As you can see, he's got a variety of little greeblies on his body and rods attached to his arms that would be very fragile. (On the Hasbro version, they omitted some these and made the other ones considerably thicker than they would actually be if they were in scale.) On the GG and Koto pieces, some parts (like the spike at the top of his head or at his collar) are about as thin as a pencil lead. You can get away with that on a static statue, but I could see a lot of breakage occurring unless they get creative. I'm sure there's a solution, but this is probably one reason that we haven't seen the droids yet.