The average pair of eyes can't tell 720p from 1080p or 30 fps from 60 fps, according to various tests I've seen. The difference between them, if the rumored specs are even correct, isn't even enough for either of those differences. You think the average person is going to pick out much lesser differences than those? You think developers are going to bother to make big differences between them when they could just make one set of assets and have it run a little faster on one or the other? The lowest hanging fruit is probably using the extra RAM on the Xbox720s for more textures, ect, and the lowest hanging fruit is going to be the one picked most.
Somewhere you appear to have equated 50% more shader flops to being overall exactly 50% more powerful. Nothing could be further from the truth. Shifty's 20% guess is much closer to reality. In a time of diminishing returns on graphics, when it takes 10x the processing power to make something look 2-3x better, the difference between them will be practically nothing. Except that Microsoft is a quantum leap ahead on developer support, software and relations.
I never said explicitly 50% more powerful.
Realistically, I'm expecting a sub/choppy 30fps title on Durango - and roughly the same title on Orbis, but running 30+fps smoothly. Perhaps higher res, perhaps with additional effects on Orbis due to additional compute resources ...