I find the ride of the Pegasus 31 to be a little lower to the ground than past versions, and the shoe is lighter and more flexible than the Pegasus 30. I've never concerned myself with stack height, but I didn't notice any achilles pain or anything going to the 31 from the 30. To me, there's less cushion in the 31, but that's not always a bad thing.
If you're a longtime wearer of the Pegasus you'll probably be fine in the 31. You're probably used to slight variations between different versions. If the Pegasus 30 was your first Pegasus and you fell in love with it, you might not like the 31 so much.
In the continuum of running shoes, the 31 is more like lightweight cushioned shoes like the Brooks Launch and Adidas Boston and the 30 is more like mid-range cushioned shoes like the Asics Cumulus or Adidas Supernova Glide.