ZMA Controller described a TCAS RA event when no traffic was observed until changing to the DARC system which did display a target; the reporter noting the back up DARC RADAR system is superior to the primary.

Narrative

A Dash 8 was a MYGF departure cleared on BR68V. The aircraft reported on at 6;000 FT level east of MUNRO and was called RADAR by the R-side. When the Dash 8 got near the MUNRO Intersection the aircraft said they were responding to an RA and they climbed to 6;600 FT MSL. At this time we showed no traffic on the RADAR scope that would be traffic. After the Dash 8 said he was clear of the traffic; I asked the R-Side to switch to DARC mode and then we saw the traffic at 5;700 FT MSL just northwest of MUNRO already past the Dash 8. When we switched back to the Primary RADAR we saw the traffic pop up 3 miles east of MUNRO for a few hits then it disappeared again. This is not the first time that our backup system has had better RADAR then our primary system. This happens all the time but is never fixed. Its a real safety issue that we have to always switch between our DARC and primary RADAR system to see limited targets. This is a known problem and from what I can remember we were told that it will not be fixed till ERAM is being used. It's not a safe situation and it needs to be addressed.