PBI Controller described a conflict event when IFR departure traffic had to be vectored off the SID to avoid descending VFR traffic; the reporter suggesting PBI be given a Class B to preclude future similar type events.

Narrative

Aircraft X departed PBI and climbed to 12;000 FT. Aircraft Y; 15 miles east of PBI on a 090 heading talking to MIA center. I noticed a VFR tag at 7;500 FT that had dropped off passing over the RADAR antenna and stopped Aircraft X at 7;000 FT to avoid traffic. Aircraft X was on the IVNKA1 RNAV departure off PBI going to ZZZ. I called the traffic. He had the VFR on TCAS. The VFR then turned and started descending rapidly into Aircraft X. I immediately took action by turning Aircraft X off the RNAV SID to a 090 heading to avoid the VFR aircraft descending into him to his left. In turning him; I was basically splitting the difference between the VFR and the IFR Aircraft Y East bound to MYAF already on ZMA climbing out of 6;000 FT for 7;000 FT on a 090 heading. Because Aircraft X was also climbing out of 6;000 FT for 7;000 FT; I believe I may have had less than 3 miles separation between him and Aircraft Y. I knew they were separated as did the pilot of Aircraft X. I chose the safer of 2 situations knowing I would have a loss of separation. My only recommendation would be to give PBI a Class Bravo airspace based on the amount of IFR and VFR traffic they work. Having VFR pilots not talking to anyone and able to climb and descend as they wish in an airspace this busy is unsafe and a disservice to the flying public.