hy are you doing mp3? mp3 is the worst by far, no matter what bitrate.
if you have equipment that supports it (or plan on getting equipment that supports it), do AAC or even better AAC-HE.
If you equipment doesn't support AAC, I would even almost say WMA. WMA quality is pretty damn close to AAC (at least WMA10), the only downside is their styupid user license which is why you don't see WMA transcoders (you can't legally convert WMA to any other format natively. you have to burn to CDA and then recode).
but yeah, there is really no reason to be using MP3 these days.
and cbr is a huge waste of space and doesn't sound any better.