a) short fourcc header for every command with command type and data
length. Therefore no command separator and no newline/space/or_whatever
escaping is needed (I know, the byte-order differences on various OSes but
hey, sockets implementation deals with the same problem and it's working
fine... htons, ntohl etc...  what's faster? Swapping of two bytes from the
header or parsing the data for [end] separator? Not all messages are sent
in one piece...)

IMHO, a) is faster than b) and the escape thing is fairly important, especially if we plan to use UTF-16 (no problem with UTF-8).