As a Crash-Rescue firefighter we had harness cutters so we could reach in and release lap belts or harnesses if needed, but as the occupant it's going to be a lot easier and faster to just use the normal belt release vs ANY rescue tool. Do what you're used to doing, don't start changing things up in times of stress.
The odds are very slim your belt release will fail to function. I know of one case where the victim was screaming his belt wouldn't release as his float plane sank. Recovery personnel were able to release the belt in the normal manner without any problem. Stress makes even the simplest task difficult.
If you're going to use a window popper be sure to do it on the side glass, you won't be able to get thru the windshield.
I was in a bad car accident around Christmas 2 years ago and was trapped in the car. I hydroplaned of the highway and t-boned a tree on the drivers side. I was pinned between the door, steering column and center console, I broke my pelvis and shattered my left elbow. I could not reach my seat belt release on my right side with my right arm, and was trapped in my car for 30 minutes.
I called 911, no passer by stopped to help, and luckily I didn't pass out.
Even after the fireman and EMT used the jaws of life on the door and removed the roof they still couldn't get me out. I had to "wiggle" my my out of the car, thank god I was in shock and didn't feel it as much as I should.
I now have side air bags in my car and a seat belt cutter, though I may get that combo thing.