3+ is alot better. If possible, rip a track from CD at Atrac3 66kbps, and then same track at Atrac3+ 64kbps. Then Listen to them on the computer or stereo or HD5 and I can almost guarantee that the 64 will sound stacks better.
I wouldn't go as far as saying 64 is as good as 132, its not quite. But I would probably go for 64 anyway, as the battery will last stacks longer plus you can fit more songs on.
For some reason I have never liked or wanted to use Atrac3, maybe because I know that it could be better using '+', its almost like using old technology, well feels like it anyway, its like it makes it sound worse in your mind simply because its not +, but its just a mental thing, It does actually sound good.
Anyway, all our problems would be solved if only Sony made firmware to make all the many new Atrac3+ bitrates COmpatible!!!