Question - why is either of those options better than a mirror?
As a supplemental system the advantages would include redundancy, lack of a blind spot, and better situational awareness even when you're not planning to change lanes.
With a mirror I'd have to be constantly checking which not only takes my eyes off the road ahead, it makes the ride much less enjoyable.
It's like blind spot monitoring and rear cameras on cars. I'd been driving without those for decades, but now that I have them I wouldn't buy a car without them.
The Varia radar also connects to the Wahoo bike computer, which is great.
> Question - why is either of those options better than a mirror?
Where are you going to put a rearview mirror on a {bicycle?, motorcycle?}? Sure you can have mirrors for the sides for lane changes, but behind you?
At most, when I'm at a red light, I lean over slightly to make sure the next person is actually going to stop and not rear-end me. Otherwise, while in motion, I have never felt the need for a rearview mirror on any form of two-wheeled vehicle.