keep in mind that the higher bitrates drain the battery faster. i have a 3G from last spring and it is about 5 hours now, but if I play 128 AAC that can go up to 7 or so.
I also heard AAC was more efficient so I recently reripped al my stuff to 128 AAC and I noticed a difference in battery, but hardly in quality, to my 192 mp3s. I mean, what's the difference if your mowing the lawn? It's not like you are in a concert hall or something 😉