I took some pictures of this little guy with his bigger Premium Format counterpart. Comparing the two I think they did a better job on the Premium Format. The dome has no seams, the weathering is better and more natural, the paint job is a bit more accurate, it doesn't have bit's opening at the slightest touch, it has metal hoses attached to the legs, the third leg looks better out and the lighting sequencing and effect looks much nicer. I tried to do a video of the lighting difference but it didn't turn out (only have a crappy mobile phone camera and the glare washed out the subtlety of the effects). If they had shrunk down the PF this would have been a much better figure in my opinion.
That said, I still really like my sixth scale R2. He does have a lot more posing options which is a major plus. His third leg when retracted goes in better as well, the touch panel is much nicer then the push button of the PF and the restraining bolt can actually be put in the correct place. My major gripe with the sixth scale one is really the seams on the dome. The one at the front stands out like dogs balls. I can't understand "it's not noticeable" comments because it's plain as day in most pictures and in hand. As fas as I can see there is no excuse for it. If there is some reason it was unavoidable it really should have been at the side and not visible from front viewing. I am considering a second - one to pose with Hot Toys Bespin Luke and the other with the SS C3PO.