The PS4 seems to be doing just about everything right from a gamer's perspective, but that alone doesn't guarantee market success.
I imagine that if Microsoft matches the PS4 in features/tech with the 720, it will be the best selling system in NA for the 8th gen. The 360 has built up a brand name in that region that will be really tough to beat. Meanwhile, Nintendo has squandered their lead with the Wii U and hasn't been making much effort to recover. While Nintendo will inevitably increase sales with a price drop and their usual lineup (3D Mario, Smash Bros., Zelda, sequels to Wii titles), they likely won't have substantial 3rd party support and I doubt they'll be able to pull anywhere near the same casual audience they got early on with the Wii.
While consoles can reverse their fortunes later on in the generation, it's rare for a system with anemic initial sales to be on top by the end of the generation. However, the PS3 might be able to pull it off, so you never know. If I had to predict the standings by the end of the 8th gen, it would be:
NA region:
1. Xbox 720
2. PS4
3. Wii U
Japan:
1. PS4
2. Wii U
3. Xbox 720
EU:
1. PS4
2. Xbox 720
3. Wii U
But frankly, anything can chance. We don't know enough about either the PS4 or the 720 to make any declarative judgments.
