If you only care about graphics, get a PS4. Of the two, it is simply the most powerful console (though a mid-end PC or better will easily beat it. However as we have seen with Ubisoft games and more recently, Arkham Knight, PC game ports are more likely to be unoptimized and buggy if not outright broken.) PS4 is the most popular console at the moment and will probably have the superior ports of multiplatform games. It also has some great exclusives like Uncharted, Gran Turismo, Infamous, and LittleBigPlanet.
If you don't mind the Microsoft Ecosystem (you said about considering a Windows gaming PC), The Xbox has some very interesting features. While the graphics are technically subpar vs the PS4, most people won't really notice. The backwards compatibility means you can pick up cheap 360 games (which are on the supported games list) in addition to more recent games. The best feature is that if you get a cheap Windows 10 PC, you can use the Xbox app to stream games from the XBone to your PC, which if you have a family, you don't have to monopolize the TV if you want to play games. The Xbox doesn't have as many great exclusives this time around (though it does have Halo, Gears Of War, and Forza Racing), but it will still get all the multiplatform games, is cheaper and the current bundles (especially the Gears Of War / Ori / Rare Replay bundle) are great for the price. Plus the MS store and various retailers are giving really good deals with buy a bundle and get free games.
However, if you just want to stay with Steam, you can also try the Steamlink with the Steam Controller and stream your games to the TV