From experience of the half-dozen or so times I've been scrambled to someone still airborne in a light aircraft - stick with the comms you have, whatever it is (which includes 121.5). A SAR cab won't get comms immediately. If you change frequency it only complicates the problem - have you gone to a new freq or is it just that we are not yet close enough / high enough?
If I had a choice, 121.5 would be preferred - SAR cabs have an extra homing receiver so we can talk to other ATC agencies while homing to you on guard. But the most important thing is not to lose comms. Certainly do not leave 121.5 for some other frequency unless told to - D&D are likely to be of more use to you than just about anyone else - they are professionals specifically at emergencies, with resources at their fingertips that other agencies will have to go digging for, and they aren't working anyone else but you and your problem.
Sven