My gut feeling here is to set it to optimize for background services on a QlikView Server. The only conclusion I can draw from the terms "program" and "background service" is that a program is something that has a GUI or is using Windows forms of some kind, whilst a (background) service is just that; a service. GUI elements are even supressed for a service, so it's very much in the background, I would say.
I really think it is a strange wording from Microsoft's direction (dumbing down the terms of advanced functionality has always been a weakness in Windows Server operating systems) and I really can't find any good documentation on it.
However, I really think it has a fairly small impact, but that's just my thought on the matter, purely untested.
Edit: Also, if this has to do with prioritization (which I suspect), it will most likely only do any difference if there are both "types" of applications fighting over resources. For me it's natural in any case to consider everything running with a GUI on a server as down-prioritized, especially since the machine is in fact a "server". Not a "workstation", which would imply much more GUI. /end of rant