RK
The angle you choose is up to you, if yoou are doing two sets and it does not take you long to adjust the angle for the second set, then why not change?
The difference is that a slighty incline (10 - 15 degrees) is somewhat easier on the shoulder angle, at least that is what I felt.
But really as you increase the incline the more your shoulders get involved, and the upper chest area seems to get more work, but that essentially the difference.