This argument is ridiculous. Just like PC games look much better than their console versions the same can be done with PS4.
I consider looking better to be a "small thing". I know some people consider it a big thing but I do not (I
like things that look better, which is why I own a gaming PC, but I don't think it's a "big" thing). I consider the size of an open world and the complexity of NPC AI to be bigger things. And I expect 3rd parties will aim for the XBone first and then add tweaks to the higher end systems.
I do expect PS4 games to look better (better IQ, more particles, more stuff on screen, maybe smoother framerate in busy scenes, etc). I don't expect the difference to be enormous - especially in the first year.
I expect that console games designed for PS4 first, or PS4 exclusively, will take more advantage of the hardware and the differences will be larger.
And if it turns out that PS4 has a big market lead over the XBone and software sales are much larger on the PS4 (who knows if this will happen) then I would expect more development focus on the PS4 and a wider difference between 3rd party games on the two platforms.