My mp3s are 192 Kbit/sec VBR. It is a good compromise between space usage and quality. (I have the lossless versions so this is just for my mp3 player).
Which software are you using to create the mp3s, as my software does not ask? If you type "VBR maximum minimum" into google, one of the top hits has an interesting forum discussion about it... (But I would be inclined to leave LAME to use its defaults).
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