I rip everything to FLAC for the Squeezebox and then again as a 256 Kbs MP3 for my portable. I've never tested that rate against anything else. I just picked a number that I figured reasonable topped out the ability of a portable MP3 player and was a balance to allow for more songs in my measly 5GB player. Having a flac version also gives me more comfort when I want to re-encode back to wave to make an audio CD.
Digression....I'm really thinking I need an 80 GB Zune. Nothing bothers me more than when I buy a new CD and have to figure out what I will sacrifice to make room on my MP3 player. They are finally in Canada and at 249 for 80GB, they are probably the best priced, per GB, of the larger, full multimedia players.