The OP is essentially correct, in my opinion.
I can spell out the technical differences, when it comes to graphics, and graphics horsepower (and the PS3 often ends up looking better in such a list, merely because its long, despite all the items not necessarily having a large impact on the user's perception), but the bottom line is that the two are so close as to practically be indistinguishable from a realistic standpoint (which includes budget, available workforce skill, market share, etc.)
Basically the X360 excels in any app which relies heavily on pixel shaders, and lots of textures, but isn't too dependant upon the vertex shader pipeline, or animation, lighting, and skinning beforehand, and the PS3... is the opposite -- better when the app is vertex, lighting, or animation heavy, and not as good if the load is on the pixel shaders.
It takes some skill to wrangle the PS3's SPUs to take all the vert/skinning/lighting/culling/animation/etc work off the PPU and the RSX's vertex pipes, but once that's done, its pretty cool -- and of course you've gone and created an app that isn't portable to the 360 in the process, making it a 1st/2nd party only thing to do, to some extent. This is why 1st party Sony devs can claim "only possible on PS3" -- yep, because the app was designed to work with the PS3, and porting it to the X360 would require enough changes that it wouldn't be the same game, even though it might look better in some ways, despite looking worse in others (sometimes it really is just impossible though, as in the animation-heavy cases).
Similarly, its pretty easy to harness the power of the flexible 360 GPU, and make it work some wonders, as far as pixel processing goes... but then you've got a game which, basically, isn't portable to the PS3 without serious sacrifices -- i.e. exclusive titles only. This is why devs of fill- and texel-heavy games (like Guitar Hero, Rock Band, etc.) complain about the PS3 not having the fill they want (because that's most of what the graphics for those games is), and its why 360->PS3 ports tend to have lower rez, or missing textures, or they are lacking normal mapping, etc. Again, some 360 games just cannot make the transition to the PS3 without some loss, and thus it might be possible to claim "only possible on X360", although I can't actually recall any X360 devs making that kind of claim.
In the end, these differences are so outweighed by developer skill as to make them near meaningless.
It'll really be something to see what the next gen brings us, given the lessons of this generation.
