B738 PIC DISREGARDS TCASII RA FROM A DHC6 ON PARALLEL RWY 19 APCH AT LAS BECAUSE OF SEQUENCING BY L30. AFTER LNDG; FO QUESTIONS CAPT'S JUDGEMENT.

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WIND WAS 190-200 DEGS AT 20-25 KTS. WE WERE DSNDING ON THE RWY 25L LOC. APCH OFFERED RWY 19L DUE TO WIND. WE ACCEPTED THE OFFER. ABOUT 10-15 NM FROM LAS; CTLR BEGAN TO VECTOR US; AND SHORTLY SLOWED US TO 170 KTS. AS WE WERE VECTORED TO THE L BASE; I NOTICED A TWIN OTTER LOW AND BELOW US ON THE APPROX SAME HDG. I SHORTLY LOST SIGHT OF THE OTTER BELOW. CTLR STEP-DSNDED US ABOVE THE OTTER. THE CTLR CALLED THE POS OF THE OTTER AS 1 - 2 O'CLOCK AT 3500 FT. WE DID NOT RPT IT IN SIGHT AT THAT TIME. WE WERE CLOSE IN AND HIGH. FINALLY; THE CTLR TOLD US TO JOIN THE FINAL ON THE ASSIGNED HDG (270 DEGS; I THINK) AND MAINTAIN 4000 FT MSL (UNTIL ON FINAL; CLRED VISUAL APCH) DUE TO WBOUND OTTER AT 3500 FT BELOW. I COMPLIED. WE WERE SOMEWHAT HIGH; SO I STARTED IDLE DSCNT AS SOON AS I TURNED FINAL. TCASII IMMEDIATELY GAVE US AN RA TO PULL UP ON CLB. I DID NOT COMPLY WITH THE TCASII RA BECAUSE THE OTTER POS CALLS HAD BEEN N OF US AND THE OTTER HAD RPTED US IN SIGHT AND HAD BEEN TOLD TO MAINTAIN VISUAL SEPARATION. NOTHING WAS SAID ON THE RADIO BY ANYONE. AFTER LNDG; MY FO SAID HE THOUGHT I SHOULD HAVE COMPLIED WITH THE RA. IN HINDSIGHT; HE WAS RIGHT. WHAT IF THE RA HAD BEEN FROM ONE OF THE MANY HELIS OR ANOTHER ACFT IN THE AREA? A GAR WOULD HAVE BEEN BETTER THAN ANY RISK OF COLLISION.