So if we're in agreement that there are no Constitutional issues here then the question is who would be more effective - an elected DA who works daily with the officers he'd have to indict, or an appointed DA who may be subject to political pressure.
First you have to realize that elected DAs are just as subject to political pressures are appointed ones. Maybe more so as an elected DA may treat high profile cases differently because they affect his chances of re-election. Secondly the local DA works
day in and day out with the officers he may have to indict. The potential conflict of interest should be obvious. So while a special prosecutor may been less than perfect my opinion would be that the chances of a just outcome are much greater with a special prosecutor than with the one who is part of the system he would have to investigate.
John Keenan, now a Federal District Judge in New York, was one of the special prosecutors who handled NYPD cases back in the day. He was almost universally considered a fair and impartial prosecutor.
Edit: Interesting debate on the topic in the New York Times recently:
Do Cases Like Eric Garner's Require a Special Prosecutor? - Room for Debate - NYTimes.com