Well, it depends on what you want from phone.
Sony is the best phone with camera quality, i had Xperia Satio model before, it had 12 pixels for that years was really good, best off all. Now i don`t know, how are there.
Samsung becomes like an iphone, i mean they are always challenging with Apple. And price is almost the same like Apple has.
Even new models like S6 Edje, with curved screen, i don`t like this model. But S6 normal model is also good, camera is also good.
Both are running Android system.
About cpu, S6 has better specs, it has DDR4 grade, and 3gb RAM. Rather Xperia has DDR3, 2gb.
This year i will choose: Samsung S6