Heat flux or open boundary

Hello all,

I am trying to modell a reactor and run a "Heat transfer in solids" analysis. The whole system is insulated except from the cap. So my question is which boundary condition should I select for the cap domain:

1)Heat Flux inserting the heat transfer coefficient between steel-air or
2)Open Boundary inserting just the exterior temp

Ι have runned both but in the second the loses are clearly bigger despite that this choice seems better to me but I'm not sure if I have understand it correctly as I 'm totally new to the use of the programm.

Thanks in advance