Writer or OutputStream?

An OutputStream is a byte-oriented stream. Any text you write has to be encoded as bytes using some encoding (most commonly ISO-8859-1 or UTF-8). A Writer is a character-oriented stream that may or may not internally encode characters as bytes, depending on what it is writing to.

EDIT If you are designing a library, then if you provide an OutputStream-oriented interface to which text is to be written, you really should provide client classes the ability to control the encoding to be used.

Text? Writer. It is intended for handling characters, honors encoding.

Stream/array of bytes? OutputStream. Works on raw bytes, has no notion of characters, encodings, strings, etc.