I don't think we're gonna see sellouts on the XB1 and PS4, they might enjoy marginally more success than Wii U but they'll struggle for a few years at least. The biggest problem is price, average consumers are the ones who make or break a video game console's success and average consumers care about price first. Part of the success of Wii was the motion controls and the casual friendly appeal of it, but another part of it was the cheap launch price, especially in comparison to PS3's $500-600 launch price and the 360's $400 price at the time.
At this point in time, new consoles at $400-500 isn't going to appeal to the casual market particularly in light of the fact that all the big games (Call of Duty, Madden, Fifa) are coming out on the current gen as well. There isn't enough of a hook there to convince people they need to spend that money right now. You'll get the diehards of course, just like Nintendo got their diehard fanbase to come out, but you won't convince the casual 3-4 game a year buyer they need to upgrade at that price yet. The issue though is that we're getting to the point of diminished returns with consoles. We're left with basically just better graphics, yes I know there's more but from outward appearances those are minor additions(most gamers aren't going to care about streaming, kinect, lightbars, ps eye cameras, etc.). The current gen brought a lot more to the table: online, DLC, wireless standard controllers, blu ray (in the case of PS3), etc.
The economy just isn't in the same place it was in past console launches, people don't have the kind of disposable income they once did. It's turning around but for the time being people are being very skittish about big purchases, particularly when it's for purely entertainment purposes. Sure the iPad is selling well but people are buying those as hybrid entertainment/productivity devices.
Sony is trying to use a lower price to attract casuals and Microsoft is trying to use Kinect/TV features to attract casuals. I could be entirely wrong and they might have it pegged but personally I just don't feel like this will be enough to attract the casual buyers creating a Christmas-time fervor for these consoles.