14 different machines to chose from currently but ibuypower, Cyberpower PC, Alienware, Zotac will be the best selling ones since they will have the more competitive prices. $500-$600 + $100 for Windows so you get the full PC library, you pay for online on both PS4 and Xbox one so this kinda evens it out.
The current specs of ibuypower and cyberpower already stomp on on consoles and Maxwell and Pirate islands haven't even came out yet this year but when they do they could switch to that hardware or drop the prices with the current hardware.
The advantage with Windows installed besides larger library is being able to use different controllers, Xbox and PS controller will be more suitable for console oriented games, the options of controls doesn't force you to one platform like people that buy Xbox Ones or PS4s only because they like the controller better. Nvidia or intel based Steam machines will provide services like dedicated recording hardware, livestreaming and etc for free. Steam already does everything PSN and XBL do better and free then there are are other stores such as GOG.
The flexible library, you can't play titles such as Don't Starve and Outlast on Xbox One nor can you play Project Spark or Titan Fall on the PS4 however PC gets the advantage of being able to play both titles along with it's own exclusives.
In terms with the Wii U if it's Steam OS only they'd most likely be on par with it or near it. If titles like Killing Floor 2 are real than the steam machines can act as a complimentary system. Sony and M$ may have to start money hatting more PC devs to make the steam machines less appealing, try to push more "deals" for their subscription based online, they won't be able to hide behind the "teh OPTIMIZATION!" since Mantle and OpenGL with Nvidia's extensions are bringing more console style optimization, not really seeing the optimization with SteamOS then again it only supported AMD and Intel till recently and is still going in development before steam machines can be sold. The 8GB ram turned out to be a bust so they have nothing else to hype the power of their console hardware since x86 is familar. Microsoft is trying to claim cloud will make graphics better but that means in order to make the game prettier you'll need to be online.
So what exactly will the PS4, Xbox One, and Wii U whip out to counter attack all this? Uncharted, Infamous, and Halo sure but the Wii U is too spaced out in exclusive offerings, Nintendo is putting too much effort into the 3DS.