The reason why people think Xbox is better than a PS3 is because Xbox Live is superior and they have great graphics compared to PS3. However, each of the consoles has features that make it special than the other and therefore I think both are great devices. I think that people assume Xbox is better than PS3 because they want to believe it in their minds that they bought the right console.
The community is better, the multiplayer servers are better, the games are better, the controller isn't awkward to use, the motion controls aren't an over-priced Nintendo knock-off, and you can play Skyrim without console lag with an X-Box.
I own a PS3, Xbox360 and a Wii and I use them all differently. I agree that Xbox Live is better for multiplayer but single player games I play in my PS3 because the sound and the graphics are better on the PS3. My Wii I only play when either a new Metroid, any Mario game or Zelda games come out. Each system have their exclusivity that as a true hardcore gamer I just buy them all because I rather have them all. Not bias to either of them.