I didn't say the difference wasn't noticable, I said it wasn't much, by which I mean it is only only the difference of ~12% higher resolution compared to the over 100% higher resolution which comes from actually rendering at 1920x1080 compared to 1280x720.
Uh, Resistance has nice multisampling on everything and Gears at least has some edge selective blurring to hide the alaising a bit, while Sigma has lots of untouched jaggies all over the environments at any resolution.
I switched to component to check 1080p and it is clearly using the same spatial resolution as 1080i, which again simply isn't much more than 720p. Also, so that we have something objective to discuss here I took a couple of pictures to illustrate the completely untouched aliasing along with the difference between rendering resolutions:
http://www.sunflower.com/~kyleb/1080p.jpg
http://www.sunflower.com/~kyleb/720p.jpg
Gears has the same jaggyness or even worse (the screencaps that you provided are closed-up).
Check this one out and judge it yourself:
http://media.teamxbox.com/games/ss/1167/full-res/1162867613.jpg