The term is ulterior motives quite a number of posters on these forums make their recommendations predicated upon boosting a hardware count in favor of their preferred console. I will give you an unbiased recommendation based upon your games collection.

You do not appear to be a heavy gamer thus you have no specific need to support multiple consoles. Your nowhere near taxing your available selection on your current platform. While it is true you will miss a few major titles due to exclusivity that hardly justifies making another major investment. Console owners for the most part have learned to accept the fact that they cannot play all of the games.

That is not to say I wouldn't recommend the 360 as a replacement for your PS3 if you felt it was not meeting your needs. I know of a couple dozen posters on these forums that have about faced, and while most of them did not sell off their PS3. They usually concede its been relegated to a dust magnet. However in your case you are planning to support multiple consoles, and unless your planning to purchase over a dozen games annually it would be a terrible waste, and more to the point it just puts you in a position where you must buy two consoles next generation if both parties decide to support backward compatibility.

Your not too far along to change your mind you can still redistribute much of your investment, but you should probably decide one way of the other curbing bets is only for a hardcore enthusiast or someone with ample cash to burn.

What he said. This would also include non-inclusive games that are reportedly superior on the 360, like Bioshock (though maybe only count those as half a game). Don't forget downloadable games (like the brilliant 2D platfomer Braid) and DLC (GTA4, Fallout 3) and future releases (Mass Effect 2, Alan Wake, HALO Wars).

Waiting on the PS4 and 720 is silly. Those systems are each a minimum of 2 years away, and likely at least 3 or more*. By the time they come out, the $200-300 you were planning on spending on the 360 will be long gone.

The real question, I think, is whether you want it NOW. The big advantage to waiting a little longer is that the console will most likely get cheaper, as will its games. I got mine in August with a $40 gift card promo, which I was able to spend on games ($20 each for new Dead Rising, Assassin's Creed, $30 for Orange Box, Bio Shock and Mass Effect). And if you still have PS3 titles to go through, why not let them tide you over? The 360 games will still be there in 6 months.