Metal Gear Solid: Peace Walker was supposed to be called Metal Gear Solid 5: Peace Walker. That's about it. According to Kojima-sama, it's going to be a true successor to Metal Gear Solid 3. The fact Kojima-sama himself is working on Peace Walker instead of Metal Gear Solid: Rising explains enough. Metal Gear Solid: Peace Walker is a true sequel to the series.
Portable Ops felt the same as Metal Gear Solid 2: The Sons of Liberty, a spin-off, albeit MGS2 brought more to the continuum of the series. Nevertheless Portable Ops is a part of the main series.
I was a bit turned off after hearing about Co-op in a Metal Gear game, but it's the same with the multiplayer in Uncharted 2 At first I was like WHY!? but then I felt like why wasn't there a multiplayer in the first game! After playing the demo I feel that co-op might just be the best thing ever happened to Metal Gear.
