I don't know what 'ace in the hole' means: I know the idiom exists, but I've had to look it up. It seems to mean what I understand by 'ace up your sleeve'.
That's figuratively, anyway — the way we ordinarily use such expressions. Literally, there's a huge difference. An ace 'in the hole' comes from the way the cards are dealt, but an ace up the sleeve is one you've hidden, intending to cheat with by adding it to your hand. But the idiomatic expression 'ace up your sleeve' doesn't (to me) convey any idea of cheating. It's just a winning thing that you can produce if you need to. There's nothing morally wrong with having an ace up your sleeve (in the usual idiomatic sense).
When playing poker, the cards face down are legitimate but hidden from others view. They are "in the hole" and you own them.
An Ace up one's sleeve is a card stolen from the deck and hidden from others' view as well as use.
Producing an Ace in the hole and winning the game would be fair play. An ace up one's sleeve (and getting caught) would mean a fast death (literally).
Today, when someone has secreted some information (legal or not) we refer to him as having an Ace up his sleeve to help him achieve victory. It may or not be used/played depending on how the negotiations go. We just don't shoot them anymore if they use it.
that's exactly what i sense to be a slight difference between the two-
CID gives the explanation for an ace in the hole as
'sth other people are not aware of that can be used to your advantage when the time is right'
while the ace up your sleeve is simply:
'an advantage that other people do not know about'
so the way i understand it,the difference is rather in the intention of using the advantage (fair play or not)?
or the other option: perhaps BEING AWARE of having an advantage (ace up your sleeve) whereas ace in the hole TURNS TO BE an advantage (when all is said and done)?
goodness,i just hope someone knows what im talking about...