I also think a helmet mirror is better than the radar. Can't understand why any cyclist on the road would want to ride without a mirror, but there are still a few holdouts here who claim they can turn around and look just as well. I use them all the time in my car, had two mirrors on my motorcycles, and feel they are even more essential to safety on my bike since cars are passing me much more often.
A couple of my buddies don't use mirrors, but now have bought the Garmin radar units. I can see the value of the radar as an early-warning, to tell me to check my mirror to determine that the car behind sees me and is moving over to pass. I'd think the radar could enhance safety combined with a helmet mirror, but for me they aren't a replacement. I'd say learn to use a mirror first.
And of course a bright flashing light is still needed. The DiNotte is popular here. It's pricey, but they are bright enough to be seen from 1/4 mile away on a sunny day, and seem to be very reliable and long-lasting. I've had mine for 6 years without any problems.