I made a thread about the same topic about 10 months ago, asking why the generally held believe was that the Wii U would make a better secondary system for anyone that already owns a PS4/X1 than a X1/PS4 would. Back then people were pretty adamant about the Wii U being better because of different exclusives and getting more exclusives. A lot of things have changed since then, so I thought I'd open the question for discussion again. Keep in mind that I'm again talking about the whole generation and not just right now, so future games should be put into consideration as well.
For anyone that wants to catch up, here's the old thread: http://gamrconnect.vgchartz.com/thread.php?id=188942&page=1
My personal opinion: When I made the first thread, I was of the opinion that the X1/PS4 could make equally good secondary system as the Wii U can, saying that all consoles would get a more or less equal amount of worthwile exclusives at the end of the gen, which would bring the whole thing ultimately down to taste. Since then I have changed my opinion and think that the X1/PS4 are both better secondary system then the Wii U and that they are both going to have more worthwile exclusives than the Wii U in their life.
Keep in mind that in this case I mean "not on the other system" when talking about exclusives, so it doesn't matter if a game is on PC or handheld, because the scenario is that you have one home console and are looking for a secondary system to complement it, not that you have one home console and a gaming PC and are looking for another console.