There is probably no difference because the buffer sizes for BufferedWriter and BufferedOutputStream are the same (8k in JDK 1.5.0). Wrapping a buffered stream with a buffered writer would not get you any positive benefit for the same reason. Since tweaking IO throughput is very application-dependent, you should look at Java Platform Performance - IO for the details on IO performance and how to measure and test it.
However if you are using a charset where more than one byte is frequently converted to a single char, then the boundaries of the two buffers aren't going to match up so nicely any more. Whether this matters I have no idea, though.
Personally I would expect that a buffer between disk and memory would produce more benefits than a buffer between memory and memory. But then I distrust that kind of "explanation", so I would produce the "It depends, try it and see" sort of answer too.