Air Carrier on Visual Approach to SFO Runway 28L experienced a TCAS RA after maneuvering slightly to adjust position with parallel traffic as per Tower instructions.

Narrative

TCAS RA 'Descend' on night Visual Approach at approximately 1;000 feet AGL.TRACON instructed us to maintain visual separation with B757 on parallel converging Visual Approach; but also to maintain 180 knots to 5.0 DME. The result was to catch up to abeam the other aircraft at approximately 1;000 feet AGL. The Localizer DME was between 5 and 6 miles at this point. Upon changeover to ATCT we were instructed to not pass the other aircraft and circle to the South if necessary. The PF made a very gradual maneuver to the South of the localizer and while re-converging with the centerline; both aircraft apparently received TCAS RA' simultaneously. Our RA was to descend. Little or no maneuver was necessary to stay out of the RA warning area on the EADI. The parallel aircraft announced 'going around; TCAS.: I decided it was safer to continue rather than go-around and the PF continued the approach to a normal touchdown. An otherwise low workload night Visual Approach appeared to be poorly handled by TRACON. Although both aircraft were in visual contact with each other; the clearances issued put the aircraft on converging trajectories inside the FAF. In retrospect; I should have selected TA only on the TCAS and/or rejected the clearance to maintain speed. As an additional factor; there may be a discrepancy between the DME source that crews are using for these speed clearances versus what TRACON is expecting. We were referencing the Localizer DME which is different than a five mile final and different from 5.0 DME from the SFO VOR.