Initial observation is that the new S90 has more boron/ultra high strength steel than the new xc90. Look underneath the driver seat and you will see a red cross bar (boron) in the floor of the S90, but this is orange (extra high strenth steel) in the XC90. This upgrade to boron in the S90 (if the depiction is accurate) may further improve side impact crash test performance/safety. Otherwise we are talking about essentially the same structure.
Like all things, Boron steel is good in its place, which is not everywhere. It is stiff but also brittle, so a higher percentage does not necessarily translate into greater safety. A car made completely of boron steel would probably be too stiff and crack-prone to use.