Incline vs. flat:
There are so many variables to bench pressing, changing the angle of your spine in relation to the extended arm is only the most obvious factor.
The bottom line with either movement is how you perform it relative to:
1)the position of the body (in specific the arch of the spine and position of the shoulder blades ).
2)the mechanics of the decent of the bar (eccentric phase), specifically the position of your elbows, and where the bar is placed at the bottom of the movement.
3)how you initiate the concentric phase (what muscles you use to start motion of the bar off the chest).
4)at what point in the ROM you again switch the eccentric phase
5)the tempo you use in performing the movement
6) breaking the concentric/eccentric cycle or going for continuous tension
without getting more technical, doing either movement is better than doing nothing, but the movement that is best for you is based more on your goals than on the the angle of you spine.