Keep in mind that sound quality is:

a) Subjective
b) Hard to determine at bit rates above 128
c) Dependent on the encoder.
d) Dependent on the music being encoded.

Here's what I can tell you: I know that I personally can't tell the difference between a VBR file that averages out to 150kbps, and a CBR file at 256kbps, assuming both were done with good encoders with all the proper quality settings.

With that said, all of my recent rips have been done at 256k CBR because I've got the space on the player and I want to ensure to myself that I never hear any artifacts no matter how slight. My ear stops noticing the artifacts above 160k CBR, so I just do 256 as overkill.

I would do maximum-quality VBR, but I've been using the Rio Central for all my ripping lately and it doesn't do VBR.