Two different ways of changing BG levels - which is better, when & why? Safer? cheaper? Obviously I care about this stuff . . .
D50 is a solution of 50% Dextrose and water. It does not need to undergo any metabolism for your body to use it. It can be directly injected through an IV and fix hypoglycemia immediately. It is cheap, and safe with one big caution. Because it is a hypertonic solution it will dehydrate and cause tissue death if it escapes the vein. See
http://en.wikipedia.org/wiki/Tonicity#Hypertonicity
So to use D50 you need to be able to obtain IV access, and certain people are difficult to access for various reasons.
(There is also a risk of neurologic problems, but that is a result of vitamin deficiency mostly in alcoholics making them unable to process it properly. That can be easily avoided with a vitamin injection with the D50)
Glucagon, like it was mentioned earlier, is a hormone that is injected into muscle. A person can be trained at home to mix it and inject it relatively easily. So some diabetics have it at home. You only have to be able to hit a muscle to safely inject it.
The downside with glucagon is that it takes longer to raise the sugar, because it signals your body to release glucose from glycogen stores instead of directly putting sugar into the blood like D50 does. It also requires adequate body stores of glycogen (storage form of glucose that is in liver/ muscle). If the low sugar is from an accidentally giving yourself a few too many units of insulin those stores might be there.
But you can also have scenarios where you deplete glycogen along with the low blood sugar, so glucagon will not work in those scenarios. An example would be prolonged illness with fever. Your metabolic demands increase, and most people will not keep up with the demand and actually may consume less while ill. So you would need to give IV D50 to raise their sugar.
The USUAL process is give D50 if IV access is readily available, glucagon if not.