Ok, I don't want to downplay RDR's merits because I think it is extremely technically accomplished, but I think just having "bigger scale" (which is a debatable term here) is definitely not enough to make up for the fact that it is less technically impressive in pretty much all other areas when compared to GOW3 (maybe even scale as well depending on how you classify the term). Even the quality of the LOD and streaming system is likely less technically adept in comparison to GOW3's which is not very forgiving and demands a lot more from any given scene, whereas RDR's is extremely forgiving (when a lot is on screen it tends to just blur, focus out, or use 2D versions of backdrops). In RDR the fastest you're really going to travel is by horse (using the War Horse myself), in GOW3, you can literally get flung a half a mile to a mile or more in a matter of a few seconds, and never even see a drop in LOD quality or even notice there is one in place.
Despite the open (albeit slightly sparse) scale of RDR, I'm willing to guess that GOW3 at it's highest points has more on screen at any one time too, on top of pushing hardware a lot further than RDR. Scale doesn't really mean much depending on other factors. You could technically just have tonnes of scale with lesser quality textures, shadows, lighting etc. Which is basically the case here (in comparison to GOW3, compared to most games RDR is more accomplished.). And with respect to scale, it's easy to forget that GOW3 also has gigantic open areas and has you fighting against mile high giant Titans. Again, in dynamic ways. Just because it's not a sandbox game does not mean it doesn't have as much scale at any given time.
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Again, I'm not downplaying RDR. It was my no 3 choice for most technically impressive for 2010.