Belum siap? I was always taught that saying tidak was definitive, and that it was more polite to say belum as it meant you were not ready yet. Such as belum makan or belum tidur etc...
It can be more polite to use "belum" instead of "tidak" (depending on context).
However, these two words have different meanings and cannot be used as synonyms.
"tidak siap" = not ready.
"belum siap" = not yet ready.
So belum would be better?