Clearly the sites that host comparisons see a lot of traffic, and generate a lot of attention. Fanboys love the idea their bias might have some basis in truth.
It's all pretty ugly.
The simple fact is the PS3/360 are pretty darn equal and if everyone would understand that when you get something like Bayonetta it doesn't mean the PS3 itself is somehow inferior, it means the developers wrote worse code for that console.
Now, with FFXIII, we're seeing the reverse (which is rarer due to earlier release of 360) with 360 getting the weaker version. Again, this doesn't mean the 360 is inferior as a console, it means it's saddled with the weaker code.
All these comparisons and games have proven is that, so far, the PS3 has received a greater number of poorer coded games but the 360 isn't immune to the same problem if the developer actually leads on PS3 first then produces a rushed port.
That's it. What they tell you is all about the developer, how much effort they put in, and which console may have received 'the rush job' but they should never be used to extrapolate anything about the consoles themselves, which is of course what most people do.
EDIT: now, as others have said, in theory the potential good reason for comparisons is to chose versions if you have multiple machines. The problem is I don't believe these comparisons really exist for that reason. That's the excuse for them to exist - although it is a valid reason. The goal is clearly to drive hits, marketing traffic and they are mainly taken all out of context.
In theory, if developers did an equally good job as they should professionaly, then comparisons would be irrelevant as they would have nothing really to note, other than minor differences of rendering colours, etc.
Try to be reasonable... its easier than you think...
