I have a set criteria. Obviously, I'll get whatever first-party games I can for either console, but for third-parties - not counting exclusives - it depends.
If it's a new game in a existing series, I'll play it on a platform that has more or less been attached to that series for me since the start. One example is Assassin's Creed. You can bet I'll pick up Unity on PS4 and Rogue on PS3 as I've played all the Assassin's Creed games on PlayStation.
Likewise, the next Mass Effect on Xbox One. Although I have the Shepard trilogy on Xbox 360, PS3 and PC, I've always considered my Xbox 360 to be my platform of choice in regards to Mass Effect.
(that is unless there's, say, a noticeable difference in performance graphically and/or gameplay wise in one platform over the other, in which case, I'll go for the version which runs best)
I'll double-dip some games for a game too on different platforms: GTA V is the most recent example to come to my head as I got both PS3 and Xbox 360 at launch. For the current gen version, I have the PS4 version currently pre-ordered, but only for a lack of funds to not get the Xbox One version (yet).
In regards to new IP, I'll play it by ear. But right now, my PS4 is my main console compared to Xbox 360 last-gen so it'll probably get a first refusal type thing. Not to mention, exclusive content and promotion attachment will probably play a part, so things like Watch Dogs and Destiny I've played/will play on PS4.