If Charlie’s take on the iTunes Plus songs wasn’t enough for you, then check out our own Wilson Rothman’s piece in the NYT about the same subject.
https://gizmodo.com/listening-test-compares-itunes-plus-to-itunes-128kbps-264617
Not to spoil anything, but Wilson and his two test partners could hear a difference between 128kbps and 256kbps on his medium to medium-high end speaker setup. However, if you’ve got a $50 speaker setup running from off your motherboard’s sound card, you probably won’t.