I've used both but currently I'm relying on handlebar end mirrors.

The nice thing about a helmet or eyeglass mirror is that you can direct the range of view by moving your head. The bad thing is you have to train your brain to see the little image in the mirror. It sometimes helps for me to close one eye. Once you get used to seeing the image in the mirror, it isn't a problem.

The nice thing about handlebar mirrors is that the image is a lot bigger. You can steer the view to the rear a little bit (but not very darn much) by moving the handlebars. If I pass another rider while riding my recumbent I have to get pretty far ahead of them before I can see them and know that it's safe to pull back over.

With either style there are variations in mounting and adjustment. The better ones are much more solid and well worth any extra cost. My favorites have been Take-A-Look eyeglass mirrors and Mirracycle handlebar mirrors. Some time soon I plan to buy some additional Mirracycle mirrors to mount on the other side of my recumbent handlebars.