A long shot is stronger and has more caffeine due to the longer extraction time. But it also has a less intense flavor and less caffeine per ounce because of the volume of water in the drink.
No, a ristretto shot consists only of water and dissolved solids from ground coffee. It does have a sweeter flavor profile and less bitterness though.
Yes, you can use identical coffee beans.
Less extraction means less caffeine. A ristretto shot has a little bit less caffeine than a regular shot of espresso.
If you a more intense flavor or a smaller shot, go for the ristretto coffee. If you want a larger drink, slightly more caffeine, or less crema, try the long shot.