It would have to be a rather large disparity for the install base to become the deciding factor. And I think the popularity of Kinect will be more of a deciding factor than people think. Remember that the Wii was by far the most popular console of the generation.
What it comes down to is which console will have the largest install base. You really think that publishers/developers will ignore the PS4 if they sell 3-4x more consoles than the Xbox One? Not a chance in hell. Plus all of the huge studios are already on board with the PS4, including Bungie which jumped ship from Microsoft.
Lastly, the vast majority of the best exclusives come from first party studios. That goes for all three consoles, but even more so for Nintendo and Sony. If Microsoft doesn't retract their policies this is going to be a barn burner in Sony's favor.