Xbox Pros:- Game selection
- Better multiplayer (goes both ways, see below)
- Older, so more users (goes both ways, see below)
- System link (hook up a bunch of Xboxs, LAN style)
Xbox Cons:- Ancient hardware standard
- Bad/fat controller (in my opinion)
- Prone to RROD (Jasper not as prone)
- Pay for premium services (i.e. multiplayer)
PS3 Pros:- High tech (US military uses a bunch of PS3s for supercomputing clouds)
- Able to be self repaired if YLOD happens, which is rare (reflow)
- Slim variants have drastically reduced heat
- FFv13.*
- Multiplayer is free, premium services optional (PS+)
- Can function as a blu-ray player
* Seriously. I know 6 Xbox owners caving in and buying a PS3 when this comes out. Granted, this might be an outlier condition, but still.
PS3 Cons:- Not as many games
- Difficult to code for, which means games are less likely to be PS3 exclusive
- As far as I know, no system link capabilities
- Fewer exclusive titles
Entering the seventh generation console market so late has its disadvantages. I believe PS4 is slated to be released at the earliest late 2012, which would pretty much kill the PS3. However, with the PS3 finally becoming profitable and developers still not using the full potential of it, it's possible that it could be as late as 2015.
The main deciding factor for me were the following:
Xbox: More friends play it
PS3: Everything else
However, that is quickly changing, since now I have more friends with a PS3 than with an Xbox. The only reason Xbox should be considered really is if you know a ton of people with one, because the game exclusivity of the Xbox is actually now diminishing.
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tl;dr version: Predicted PS3 sales > Predicted Xbox sales in 2010