Quality exclusives are the key reason you would want a console over another and in the long run, PS4 is likely to gonna win over XBONE due to their having access to more studios that produce exclusives. Support for previous gen games is simply not enough to sell a console. Early generations of PS3 also had support for previous gen, but that was never a system selling feature.
In any case, it is highly unlikely that PS4 would get backward compatibility through emulation. Definitely not the titles like Uncharted or God of War which made full utilization of PS3 cell architecture. XBOX 360 used a simple 3 core Power PC RISC based CPU. There should be no surprises in emulating it on a x86 based CPU. PS3 while was also a RISC based CPU still had a unique architecture which I guess would not be easy to emulate.
As for hardware of new gen, while on paper, specs look similar for XBONE and PS4, there must be a reason why multi platform titles are often rendered at lower native resolution on XBONE compared to PS4. That still would not be enough to overcome the technical constraints of PS3 on a x86 CPU.