Well first off I can tell you, the Xbox is a million times more reliable than it was when they were croaking left and right. I got my Xbox in November 07, just after they added in some nanoprocessor or something in to help make it run cooler. It did break down, but it wasn't 3 red rings, and they reset the warrenty on it, including the 3 year RROD coverage. And it was about one year after I got it. I've used it a lot and compared to the early rushed models that is like a lifetime longer. There are also quite a few ways to play online without the adapter - depends on how close your PC is to your console. Mine is right next to it so I just use the PC connection. Actually, the Xbox website itself tells you a few ways to get onto Xbox Live without the adapter. And yea peripherals are way overpriced, but so are Sony's.
But really it depends on the games I guess, and yea you're only gonna get MGS and such through the PS3 so I guess you'll go that way. Plus, blu-ray beat HDVD or whatever that other HD type M$ was trying to trumpet.