which way should airbrakes face?

[not enough fuel]

when i'm aerobraking in kerbin's atmosphere to lower my apoapsis, should the aerobrakes open towards retrograde?

 

 

or should they open away from the entry?

 

 

[OHara]

If you want to make them work like aircraft airbrakes, use the orientation your second picture.  

Actuators don't fail in KSP, but the way that part looks it is easy to imagine the actuator failing, so you might be more comfortable if the airbrakes look like they would retract if they fail.  Or, in your case you might prefer them to stay deployed if they fail so might want them in the orientation in your first picture.

The orientation makes very little difference in their drag, as implemented in KSP, for this particular part.

[jimmymcgoochie]

Short answer: no difference at all.

Long answer: it depends how deep you want to go- if you like how it looks with the airbrakes deploying in one direction, use that; if you want to pretend that all the mechanical and material constraints of the real world also apply to KSP, put them so the flat surface faces the airflow as this both shields the hydraulic deployment arm from the superheated plasma of re-entry and reduces the risk of the whole thing being ripped off if it produces too much drag. (If you happen to drive a car with the door open into a lamppost, hitting it going forwards will just slam the door shut but hitting it backwards will bend it the wrong way and quite possibly rip it clean off; the same principle applies here too. Of course, this is not a good idea so please don't drive your car into a lamppost!)

[Akagi]

On 2/6/2022 at 1:17 PM, not enough fuel said:

or should they open away from the entry?

Away