I usually depend on my mirror to keep an eye on someone overtaking me, either bicycle or car. These would seem to have minimal effectiveness for watching someone as they are approaching and overtaking... But I must say, of the two the Delta looks more interesting to me, it depends on how good the view is to the left rear.
__________________
Slow Ride Cyclists of NEPA
People do not seem to realize that their opinion of the world is also a confession of character.
- Ralph Waldo Emerson