iostream vs. stdio

so, I've got EXPERT PROGRAMMERS telling me to use printf instead of cout in c++, ok but why? what is the difference (in b4 stream)

>>71
doesn't work:
$ echo -e "#include <inttypes.h>\nPRIu64"|cc -E -|tail -n1
-e #include <inttypes.h>\nPRIu64

this works:
$ echo "#include <inttypes.h>\
PRIu64"|cc -E -|tail -n1
"llu"