Looking at this again, I think the chest and overall physique on HT's Ultron Prime looks pretty good actually.
I like the King Arts figure, but I don't like how the "cheeks" look less emphasized and how they dropped certain details on the head sculpt such as the circle on his chin, the ridges on his "eyebrows" and the indent between his upper mouth (which was key in simulating his traditional jack o' lantern visage). They even added an unnecessary sculptural line on his "nose", which is completely absent on the CG render and HT's figure. Lastly, and probably the biggest one for me, is how it doesn't seem to have an articulated mouth/jaw.
That's where HT has won me over honestly.