I will get both
xb1 because i am a huge halo and forza fan and because i really like some stuff like the skype integration which i want to use when i play co-op or whatever else against/with friends and family. my internet is horrible right now but i will get better internet in december. i also like kinect in general, something which is not often to see in gaming forums
ps4 because i simply like the playstation exclusive library. i'm not a huge fan of any single sony exclusive like i am with halo (but i really love gt) and could live without any of them but the whole package of games they release in a gen is just too good. i would say i prefer some xbox exclusives above any playstation exclusive but find the whole exclusive library of playstation games better on average.
€dit: i won't buy both at launch, i believe that's what you wanted to know?