Let's put it the other way around: Backstab and Ambush have not much in common!
You use Ambush when you're in stealth, as an alternative to other openers (mainly, for me as well, Cheap Shot).
You use Backstab when you're not in stealth (otherwise you're better off using Ambush) but when you're behind your target nonetheless, i.e. in solo after a Gouge , Cheap Shot, Kidney Shot or when in a group when another character has the aggro. Backstab in an alternative to Mutilate, which also requires you to attack from behind your target (Assassination Tree, but requires 41 points in Assassination so for now you stick to Backstab). I read that some people do not use Backstab but Hemo to generate CPs faster, can't tell much more about that.
Common point between the two: you need to hit from behind the target. Apart from that, not much in common.
nostra