I have a Take-a-look mirror, the shorter version. I always ride with it. If I'm riding the local bike share bikes without my riding glasses, I really miss the rear view.
I don't actually see all that much when my head is in a normal riding position, and if I have a jacket, then half the view is blocked by folds in the jacket. I have to hold my head just right to see straight back as I'm riding.
Instead, I turn my head slightly to sweep the view behind me
. Just a slight head turn, maybe 10 or 15 degrees, pans the view from way off the left side of the road behind to way off the right. It's fast and I do it without thinking now. (Some years ago, I was walking down the hall at work, heard an odd noise behind, and instinctively turned my head slightly to scan with my non-existent mirror.)
A glasses mounted mirror gives you a narrow, but full sized view. A bar mounted mirror has to be convex to have any coverage, so everything is small, like a right side car mirror.
So, with a right side glasses mirror, you should be able to see the lane behind you with little or no head movement, and a quick head shift to see everything behind.
Field of view
I just went outside and tested my field of view. The shorter mirror should give a wider field of view, since the mirror is closer to my eye. But where I have it positioned, the outside portion is way at the edge of my vision, not as in focus and a strain to see. On the inward side, I see the edge of my ear.
The usable width is about 1.5 to 2 fists held out at arms length. Or a fist + thumb held out. Not all that wide. But very usable with small head movements.
If I'm in the drops, my view is mostly blocked anyway, unless I lift from my normal head position. On the hoods, with some effort, I can keep continuously monitoring the rear view, but it's not easy.
I'm in the habit of checking periodically to evaluate traffic behind me. Less often on quiet roads, more in busier traffic. I occasionally get startled by cars with very quiet tires if there's lots of wind noise in my ears, and I haven't been looking back much. Even with a full view style mirror with the road always in view, the rider needs to focus attention there, just like when driving, so it's not a continuous view anyway.
I'll stay in the lane's right tire track as much as possible, for less debris on the road, and no chewed up road edges. When there are cars approaching in the opposite direction, I glance back to make sure there's nothing coming up on me from the back--I try to manage the situation where the two cars pass each other right near me. So I'll either take the lane aggressively or get way over on the shoulder (if any). But usually, no cars are close behind, so I can just keep riding.
Mirrors are so nice for dealing with left turns. I can switch lanes in between groups of cars, watch that cars are slowing correctly, and make the turn smoothly.
I sometimes lead group rides, so a mirror makes keeping track of the riders behind me easy. I can even see riders still far back on a 90 degree side road before their turn. So you should be able to keep track of riders approaching on your left, using a right side mirror, until they are just a bike length or two away.
One downside of a mirror is watching those occasional "last second" drivers. They stay on a collision course in my lane until about 2 seconds from impact, then move over to pass. Grrr. They probably zoom up on cars and brake, then tailgate too. Perhaps it's better not to see?
Supplemental bar end mirror
Perhaps your best choice would be a right side eyeglass mirror, to see clearly way back down the road and off to the sides, and a left side small bar end mirror to keep track of co-riders and nearby cars as they get closer to you.
One of these styles: