Personally, I think the 2.5 looks substantially bigger (I mean, of course, we''re talking about mm here so it''s all relative).
I also never really understand the point of those demonstrations like the last one you posted, with the diamonds face-down against a white background. I appreciate that this is supposed to show the yellowness of a diamond, but when do you actually see a stone like that? Once a diamond is mounted, a number of flaws can be hidden. It depends on how much you need a "mind clean" stone.
I honestly think once the stones are mounted in a ring, 99% of people, including you possibly, won''t be able to tell the G stone from the I.
Like I said in my earlier post, when i went from my 2ct to my 2.50ct the difference is there, but not huge, especially once set. I only changed stones because of an inclusion that was bugging me.
Regarding the color- first two pics I can''t tell, the third is obvious though. I don''t get the whole low G, high I thing as that would bring them both closer to an H, and yet the difference is very obvious. In that size, there will be times depending on the lighting when you may see the color. Personally, I wouldn''t do it. But color is a personal thing and for many people size wins over color.