For self-rescue, a seatbelt cutter should in my mind be mounted in a readily accessible location in the car and not in a bag/glovebox/trunk.

One of our partner company's has a training device: a car mounted in a special rig, which allows them to turn the car upside down (or any other angle) and use it to train people to get out of it. I had the opportunity to play with it. It's quite confusing when you are hanging upside down, and since you are hanging upside down in the seatbelt, things in your pockets and belt become very hard to access. Leaning over to the glovebox; forget about it. Bag stored somewhere in the car, during a flip things start becoming flying projectiles.

Don’t under estimate the how disorientating flipping inside a vehicle is. I have also been trained twice in a helicopter crash simulator. After a 540 degree turn underwater, I keep getting out and on the surface and see the other side of the pool than I think I came out of…

A good self rescue tool should be accessible in any possible position, while remaining in the seat. It should be securely mounted (use a tool with a proper mound, Velcro is not a proper mount), be big (easier to grab, especially when you are a bit shaky) and bright (even in daylight, you get a lot less light in the car if it’s upside down). I like V shaped seatbelt cutters, because it’s much harder to accidently cut yourself. My best experience is cutting at a 45 degree angle, in a slow but steady motion, while keeping the belt tight.

I personally have a spring loaded rescue tool with seatbelt cutter mounted on my door (lifehammer evolution), one old fashion hammer with cutter on the center console (lifehammer classic, as delivered by the car dealership) and a reqme on my car keys.

(as for glass, a centre punch/spring-loaded window punch will work under water while a regular hammer type might not work under water due to the water resistance and your ability to swing it)