Basic strategy for breaking 9's on second draw

[gergery]

I find myself in situations where I need to decide whether to break a 9 and wanted some advice on what are most important considerations

example, I have 2468x and draw 1 to make a 24689. Should I keep it or break it if its:

HU and I have position he draws 2?
HU and I have position he draws 1?
HU I am out of position?
vs. 2 opponents who both draw 2?
Do your answers change to any of the above if my hand is 24789?
do your answers change if I have 23479?

thx,
-g