This question is so often!!! :-)
PS3 will win the console war, no doubt. Xbox 360 and Wii are so weak from technological point of view, that in couple of years they won't be capable of dealing with serious graphics. GTA IV, for instance, isn't even announced for Wii - this toy won't be able to cope with the game's requirements. Xbox 360 is getting GTA IV, however, the game is going to be released on several DVDs - Xbox 360 owners will have to interrupt gaming process and switch discs (Rockstar complained about this issue). Only PS3 will have it on single BD-DVD.
Let's compare PS3 and XBOX 360:
1). A third of XBOX 360 (33%) are malfunctioning right from the start. PS3 return rate is only around 0.2%. Despite some people say, that new 360 has no breakdown issues - just visit Xbox section of Yahoo answers and see for yourself an amount of complaints.
2). PS3 has 7 kernels in Cell processor, Xbox 360 -only 3.
3). PS3 uses advanced tech called Blu-Ray, one disc can store up to 25 (single layer) and dual layer 54 gb (in the future - up to 200 gb), Xbox 360 - regular dual layer DVD-9 (8.5GB). It's possible to get HD-DVD (max. - 30GB) for 360, but it'll cost you around 50$. Besides, HD-DVD had lost the format war with Blu-ray. Toshiba had stopped to manufacture them, Microsoft too.
4). Calculations per sec - PS3 - 51-billion-dot-product-operati... per second
Xbox 360 - 33.6-billion-dot-product-opera... per second
5). PS3 has full media support - USBs, built-in 4 slots card reader (except 20GB and 40GB versions), ability to use printer and so on.
Xbox 360 has USBs and only 2 memory slots.
6). PS3 supports up to 7 Bluetooth Controllers
Xbox 360 - up to 4 Controllers
7). PS3 - wi-fi included (except 20 gb version)
Xbox 360 - wi-fi will cost you around 100 $
8). From the first technical sight - graphics are stronger in 360 (it has a stronger graphics chip - Xenos), BUT PS3 can use it's Cell CPU for graphics too, which gives you better graphics in total. PS3 uses a weaker RSX chip, that was developed by Nvidia exclusively for PS3. RSX runs on 550 Mhz. It's difficult to compare their graphics, because 360 has a video chip with totally different structure and operating memory is divided differently in both consoles. Xbox isn't that versatile in dividing tasks between CPU and GPU as PS3. That's why PS3 exclusives look better than 360's exclusives.
9). Xbox 360 is widely known for so-called "red ring of death" problem - it gets overheated easily. My PS3 was running for more than 15 hours non-stop with no problems. Sometimes Xbox 360 becomes so overheated, that thermopaste on CPU melts, drips on motherboard and successfully shortcircuits it. That's why in some countries people call Xbox 360 "Coffin 360".
10). In 2008 PS3 will get more than 300 games, including Metal Gear Solid 4, The Agent and other great exclusives. Here is the good link to the future PS3 releases. http://ps3.ign.com/index/release.html......
11). PS3 online is free, Xbox 360 - you have to pay.
12). There were reports, that 360 scratches discs. No reports on that in PS3.
13). The funniest thing, that right now there are no games for PS3, that show full potential of the console. Biggest games weigh only 25 GB (Xbox 360 will need more than 3 discs even for those).
14). On PS3 you can install other OS, such as Linux Yellow Dog, Ubuntu and others. Sony officially lets you do it. There is no such thing in 360.
15). PS3 HDD can be replaced easily with any 2.5" SATA 5400 RPM HDD (used in laptops). 250 GB will cost you around 170$. In 360 you have to buy exclusive HDD, where 20GB will cost you 100$.
16).Sony expects PS3 to stay on the market for, at least, 7-10 years. Also Sony reduces price now. There is a cheaper 40GB version of PS3.
17).Sony constantly upgrades PS3 software (free updates for OS) - you get new functions and improvements.
18). PS3 controller (sixaxis) has a motion control - you move controller - game responds. Wii has it, Xbox 360 - doesn't. PS3 is getting rumble controller (Dualshock 3) on 15.04.2008, Japan already has it. On www.ebay.com I saw those controllers on sale 50$ and up (unofficial import).
19).On PS3 you can help sick people by running Folding Home project. Wii and Xbox 360 didn't even think about it.
20).New 40GB is 399$ or 399 euro (depends on the region) - it has no backwards compatibility with PS2 games, has 2 USBs instead of 4, no built-in card reader, but it's the same PS3, strong, reliable, less noisy, different cooling system. The price is friendly.
21). But, to be just, I'm not a PS3 fanboy, so (despite I own one) hear this - PS3 games (exclusives) are very hard to develop due to complicated Cell's architecture. We have to wait till developers will figure out how to develop games properly. Sometimes multiplatform games run not so good on PS3 due to the same developing issues - it's hard for developers to get a grip on such advanced technologies.
I didn't mention Wii, because it's weak and it's intended for a different market. Leave it for casual games.
Sources:
www.teamxbox.com
www.kotaku.com
www.qj.net
www.ign.com
www.wikipedia.org
www.newegg.com
www.ps3fanboy.com
www.ebay.com
and many other sources.