OK, it's still a pointless question
. . . but I'm asking it anyway.
Back on May 10 I thought it was important to say "ess-a-pee" but now I'm not so sure.
Because even though the phrase "an SAP" appears 420 times in the 391-page SAP HANA Developer Guide, most (391) of those references appear in the boilerplate phrase "...an SAP affiliate company. All rights reserved...".
In the same document the phrase "a SAP" appears 25 times, as in "a SAP HANA system" and "adding a SAP Variables clause in your select statement".
That's a score of 25 for sap, versus 30 for ess-a-pee if we count the boilerplate as only one occurrence.
Pretty much a tie, doncha think?
So I'm going with this:
Answer: You pick the pronunciation that sounds right to you, given the context: sometimes sap, sometimes ess-a-pee... or you can use one or the other all the time, your choice.
It's called "English", which in German means "Anarchy".